WEBVTT

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Today, we're talking about
immunology as you may recall.

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Let's roll.
Yes, let's roll.

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And by the way, admire
your little finger,

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because most of the planet
isn't as gifted as you are.

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Not everyone was guaranteed
so many brains as you and I,

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seriously, have been fortunate
enough to have been granted.

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Let me just finish up what we
were talking about last time,

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which was the
cell cycle.

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You may recall that we talked about
oncogenes. We talked about Ras and

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the Ras oncoprotein, which
sends out mytogenic signals.

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And, what it does, just to
summarize what we were talking about

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is it pushes cells from the
beginning of the G1 phase of the

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cell cycle up to a decision
point in the life of the cell.

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It's called the restriction point.
And here, just by way of review,

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the cell has to decide whether or
not it's going to commit itself,

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essentially irreversibly, to go
through the rest of the cell cycle,

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or whether it'll stay in G1, and
may even retreat from the cell cycle

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into G0. And Ras pushes
this decision forward.

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The retinoblastoma protein, which
we talked about the last time,

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stands as the guardian of the
gate right here, the RB protein.

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And, the retinoblastoma protein
holds this gate shut unless and

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until certain preconditions
have been satisfied,

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on which occasion the retinoblastoma
protein opens up the restriction

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point gate, and allows the cell
to go through the rest of the cell

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cycle. And now, interviewed
from this perspective,

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we can begin to understand
how hyperactivity of Ras,

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and how the inactivation of this
RB, this retinoblastoma, tumor

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suppressor protein have such
disruptive destabilizing effects on

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the proliferative
controls of the cell.

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Keep in mind, this is a negative
actor on cell proliferation.

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It's a tumor suppressor gene which
must be inactivated in many cancers.

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This is a proto-oncogene, or
an oncogene, which must become

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hyperactivated. Now, I
want to move from that into

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the topic of today, and
that's the whole issue of

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immunity. And, much of
our immunity comes from

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understanding the way we deal
with what viral infections.

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The fact is, just to cite
one arbitrary viral infection,

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relatively few people die from viral
infections these days because we can

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become immunized against them.
The first immunizations already

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began in the late 18th century,
believe it or not, when a physician

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in England called Edward Jenner
first noticed that women who had

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worked as milkmaids milking cows,
and who got a disease called cowpox

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from milking the cows seemed to be
immune to the disease of smallpox

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which was by that time realized
to be spreading in epidemic,

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so a highly infectious agent, and
one which actually killed quite

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a few people. And Jenner
intuited correctly,

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in retrospect, that the experience
of these milkmaids and their

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exposure to cowpox exposure
somehow protected them,

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gave them indeed lifelong protection
from subsequent smallpox infection.

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Subsequently to that, the
sores from the cowpox infection

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were scraped, and the exidate, the
fluid, was scratched into wounds

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of people in order to immunize them.
And so, immunization them. And so,

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immunization already began in the
1790's, taking a sore from the skin

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of a cowpox infected patient,
injecting that into the skin of

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somebody who required immunization,
and as a consequence, hoping that

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this would confer in
them lifelong protection.

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In some cases, actually
the individuals who were

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infected in that way actually came
down with smallpox or some virulent

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form of this cowpox, but
in most other cases these

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individuals actually
acquired a lifelong immunity.

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In fact, the very word vaccine,
which was used already at the time,

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comes from the Latin word
vaccinus which means a cow.

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And, in the north side of Cambridge
Common there's the Benjamin

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Waterhouse which is still there.
He was the first physician to

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introduce smallpox vaccination into
this country already in the end of

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the 18th century. If we
fast forward to a situation

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like poliovirus, we have
situations in this country

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where in the 1930's-1940's, there
were epidemics of poliovirus.

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If one began to examine who
was susceptible and who wasn't,

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it was clear that children who
were, for example, born and raised in

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middle and upper-middle class houses
were very susceptible for much of

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their lives. For example, in
southern California there were

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dramatic examples, whereas
children who grew up across

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the border in,
let's say, Tijuana,

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Mexico, rarely came down
with paralytic polio.

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Poliovirus, as one soon learned,
was a virus which infects not only

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the gastrointestinal tract, and
creates a form of mild diarrhea,

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but it may be one out of 100 persons
the virus escapes from the GI tract,

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from the gastrointestinal tract,
invades into the central nervous

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system, and actually creates
debilitating paralysis,

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most of which is not healed.
And some people have lifelong

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paralysis. Other
individuals whose paralytic

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paralysis goes away,
actually when they grow older,

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30, 40, 50 years later, they
begin once again to experience the

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paralytic symptoms that arose as
a consequence of their childhood

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infection. And at this time, one
began to try to figure out why

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children living in Tijuana,
Mexico rarely came down with

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poliovirus infections, whereas
those who grew up up north

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like, say, in southern
California did indeed do so.

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And one came to the conclusion that
the children growing up in Tijuana,

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Mexico were frequently exposed very
early in their life to contaminated

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water, sewage contaminated water,
and they as a consequence acquired a

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lifelong immunity without getting
sick, whereas children who grew up

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in rather sterile conditions further
north never had any exposure to the

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virus. And when it hit them as
young adults or as teenagers,

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it created devastating effects.
And this indicates, once again,

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that somehow one's exposure
to an infectious agent,

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historical exposure has a dramatic
effect on one's susceptibility

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to a virus. And in
the case of poliovirus,

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we're dealing here with an agent
which is much simpler than the

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retroviruses we talked
about last time like RSV.

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Poliovirus also has a
single stranded RNA genome,

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and that's encapsidated
in a proteinaceous coat,

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which is made up only of viral
proteins, so it's very simple single

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stranded RNA proteinaceous coat.
The single stranded RNA is actually

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polyadenylated at the three
prime end. So, it can serve as a

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messenger RNA. It's of
the same polarity that has

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a plus polarity, which
means that it can be

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translated immediately. I'm
distinguishing that from the

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other flavor of single stranded RNA
which could be of the complementary

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strand, which could exist because
one could make double stranded RNA

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which obviously
cannot be translated.

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So, this can serve as a messenger
RNA, and just as an aside,

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the way that poliovirus replicates
is totally different from that of

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retroviruses. What happens is that
poliovirus makes its own polymerase.

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You go from single stranded
RNA to double stranded RNA,

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i.e. the complementary copy is
made, and that in turn is used as a

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template for making more single
stranded RNA, progeny RNA.

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And note here that there's no
DNA at all involved. In fact,

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poliovirus can grow inside a
cell that has been deprived of its

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nucleus. Moreover, if one
stops nuclear DNA dependent

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transcription, that
has no effect on this.

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But you'll notice correctly that
these kind of steps here involved

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RNA dependent RNA polymerases.
And in the normal physiology of a

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cell, that such a polymerase,
such an enzyme never operates.

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And therefore, poliovirus must
make among its other proteins an RNA

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dependent RNA polymerase that
can mediate these steps: making a

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complementary copy of this strand,
to get the complementary strand, and

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then making progeny
plus strand RNAs.

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Again, it has a very simple capsid
of several viral proteins which wrap

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it up. Now, if one wants to assess
the potency of poliovirus RNA,

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one can grow it in a Petri dish.
One way of figuring out how much

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poliovirus RNA is in a solution is
to make a monolayer of cells like

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this, and then take various
dilutions of a virus stalk.

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And when one talks about a virus
stalk, one talks about a solution of

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virus particles. One
can't really see them.

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And even if one could see them
under the electron microscope,

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one wouldn't really know what
fraction of the ones you were seeing

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were actually biologically
competent. But most interestingly,

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you can take a solution
of poliovirus particles,

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place it on a monolayer of
cells here, which can be infected

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by poliovirus. In contrast
to what I told you last

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time about RSV, where an
infected cell can tolerate

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the continued presence of a
viral infection without dying,

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poliovirus is a highly cytopathic
virus, and by that I mean it

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replicates inside cells and it
kills them during the course of

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replicating inside these cells.
So, here what one has is just a

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poliovirus will infect a cell here,
and then it will begin to spread

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from that cell to neighboring
cells in the surrounds.

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And in so doing it will create,
it will erode a hole right here in

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the monolayer whose presence, a
so-called plaque, signifies the

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fact that there was an initially
infecting virus particle there that

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spreads centrifugally, that
spread outward, from the

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initially infected cell. In
detail, if you want to do this

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experiment really nicely, what
you do after you infect the

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initial monolayer of cells,
which I show here in section,

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is you put on a layer of agarose,
or something, above the infected

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monolayer, and that ensures
that if there's any viral spread,

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it will be from cell to cell spread
in a certain neighborhood rather

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than virus particles getting into
solution and swimming around all

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over the plate and affecting
cells helter-skelter.

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So here, we might want to infect
an initial cell right here in the

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monolayer, and now this agarose
will confine the subsequent spread of

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progeny virus to adjacent cells
in the area, again eroding,

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ultimately, a hole here
which we call a plaque.

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And if we count the number of
plaques, that will tell us how many

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biologically active virus particles
there were in the initial solution

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that was previously applied to that
monolayer culture of the susceptible

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cells. One could use such monolayer
cultures, in fact, to propagate

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poliovirus. And one
can take the poliovirus

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coming out of those monolayer
cultures, and actually use them to

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inject into human beings in order
to vaccinate them to confer on them

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immunity. But if you do that
with wild type poliovirus,

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then what will happen is that you
may inadvertently give that person

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whom you're attempting to immunize
a nasty poliovirus infection which

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could paralyze them,
might even kill them.

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When I was growing up,
poliovirus infections were much

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dreaded because we knew of people
who were being kept alive in iron

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lungs that breathe for them because
their autonomic nervous system had

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been destroyed by a
poliovirus infection.

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So, what happens is therefore if
one wishes to immunize somebody,

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if one wishes to vaccinate
them against poliovirus,

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one needs to inject them
with an attenuated virus,

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that is, a virus whose
ability to create disease,

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whose pathogenic abilities,
pathogenic, remember, refers to the

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ability to create disease. You
want to use an attenuated virus,

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which might be able to go into
the body. It might be able to evoke

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immunity, but it will not
create a major disease response.

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And this was dealt with
in two different ways.

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Jonas Salk, one of the pioneers of
creating a poliovirus vaccine took

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the poliovirus particles that have
been produced by culturing them on

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monolayers of monkey kidney cells,
and he treated the poliovirus

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briefly with a little
bit of formaldehyde.

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And formaldehyde, as you
may know, reacts with the

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amine groups of the ribonucleosides,
of the purines and pyrimidines, and

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therefore kills the virus. And
that kill virus, having been

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treated briefly with the
formaldehyde, the virus particle is

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still essentially intact, was
then injected into individuals.

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Alternatively,

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his deadly rival, because
they two hated each other's

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guts, Sabin decided on another
strategy for making a vaccine stalk.

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And that is, he took poliovirus
which had been passage from one cell

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culture to the next over
extended periods of time,

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over several years. So, it
had been passage in vitro.

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When we say in vitro, in this
context we mean poliovirus had been

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taken from one plate, put
into another plate of cells,

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taken to another plate of cells.
In vitro here implies in culture

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rather than the other alternative.
In vivo means that the virus is

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being passaged in a living organism.
And as it turns out, when you

00:14:43.000 --> 00:14:48.000
passage poliovirus in vitro from
passing it just from one culture of

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cells to the next, infecting
one after another in a

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serial or sequential fashion,
then the virus is selected for its

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ability to proliferate in
the cultured cells in vitro.

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But it gradually loses its ability
to create disease in vivo in that

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there is no Darwinian selection to
favor its disease causing ability.

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And consequently, virus that
Saben used wasn't inactivated with

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formaldehyde. It was a simply
attenuated because of extensive in

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vitro propagation. And
when he injected that into

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young people, very rarely did
they ever come down with poliovirus

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infection, and the virus was able
to replicate to some extent in their

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bodies without causing disease,
and to create lifelong immunity.

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I.e. such an individual was
protected from subsequent poliovirus

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infection ever again.
And, as it turned out,

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this was a very nice way of
protecting because it also turned

00:15:50.000 --> 00:15:55.000
out the monkey kidney cells that
were used to propagate the virus

00:15:55.000 --> 00:15:59.000
also contained certain
monkey viruses. For example,

00:15:59.000 --> 00:16:04.000
it came out after years that
there was a DNA tumor virus.

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We've been talking about
RNA tumor viruses until now,

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but there's a DNA tumor virus called
SV40, which is a very potent tumor

00:16:11.000 --> 00:16:15.000
virus in hamsters,
and mice, and rats.

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And it turns out that SV40 lingered.
It lurked in monkey kidney cell

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cultures. And,
nobody knew about it.

00:16:22.000 --> 00:16:26.000
Whenever you put poliovirus
into many of these cultures,

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not only did poliovirus replicate,
but all of a sudden so did SV40.

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And therefore, many of
the stalks of poliovirus

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that were used to inject people like
myself, I lived around the corner in

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Pittsburgh from Jonas Salk, and
the children in the Pittsburgh

00:16:42.000 --> 00:16:46.000
public schools were his first guinea
pigs. Many of us were exposed to

00:16:46.000 --> 00:16:50.000
poliovirus stalk which actually
had more SV40 particles in it than

00:16:50.000 --> 00:16:54.000
poliovirus particles because of
this inadvertent contamination.

00:16:54.000 --> 00:16:58.000
There probably were between 30 and
40 million people who were immunized

00:16:58.000 --> 00:17:03.000
with poliovirus and
inadvertently with SV40 virus.

00:17:03.000 --> 00:17:07.000
And one is that epidemiology in
the subsequent years to figure out

00:17:07.000 --> 00:17:11.000
whether that had led to any
increased rate of cancer because it

00:17:11.000 --> 00:17:15.000
could well have. There
could have been an epidemic

00:17:15.000 --> 00:17:19.000
of cancer in this country,
millions of people affected because

00:17:19.000 --> 00:17:23.000
of this unknown, and at
the time almost unknowable

00:17:23.000 --> 00:17:27.000
contaminating virus. As it
turns out, people who were

00:17:27.000 --> 00:17:31.000
infected with SV40 inadvertently
have no higher rates of cancer than

00:17:31.000 --> 00:17:35.000
anybody else even though this virus
is a very potently tumorogenic virus

00:17:35.000 --> 00:17:39.000
like RSV, but doing so in this
case in rodent cells and ostensibly

00:17:39.000 --> 00:17:43.000
not in humans. So,
public health measures can

00:17:43.000 --> 00:17:47.000
sometimes have unforeseen side
effects or consequences that nobody

00:17:47.000 --> 00:17:51.000
anticipates ahead of time.
Now, getting back to the whole

00:17:51.000 --> 00:17:56.000
strategy of poliovirus infection,
this raises the issue of what it was

00:17:56.000 --> 00:18:00.000
in the poliovirus that was able
to evoke the subsequent lifelong

00:18:00.000 --> 00:18:05.000
immunity. And in telling you
that, I mentioned the following.

00:18:05.000 --> 00:18:09.000
You can make antiserum or you can
make serum from an individual who's

00:18:09.000 --> 00:18:13.000
been immunized with poliovirus.
And, recollect, we mentioned this

00:18:13.000 --> 00:18:17.000
before, the way to make serum
is to allow blood to clot.

00:18:17.000 --> 00:18:21.000
The red cells and the platelets
aggregate in the bottom,

00:18:21.000 --> 00:18:25.000
and what remains on top is simply
serum. You can spin out all the

00:18:25.000 --> 00:18:30.000
residual cells and so all you get
is sort of a straw colored fluid.

00:18:30.000 --> 00:18:34.000
And that serum from an individual
who's been immunized with poliovirus

00:18:34.000 --> 00:18:38.000
stalk can actually be used to,
you can add it to the poliovirus

00:18:38.000 --> 00:18:42.000
stalk prior to adding these
virus particles to the Petri dish.

00:18:42.000 --> 00:18:46.000
So, here's the Petri dish as before,
and what you find is that if you add

00:18:46.000 --> 00:18:50.000
serum from an individual who's
been immunized to a solution of

00:18:50.000 --> 00:18:54.000
poliovirus particles, and
then you take that mixture and

00:18:54.000 --> 00:18:58.000
put them on the plate, you
no longer get any of these

00:18:58.000 --> 00:19:03.000
plaques that I
referred to before.

00:19:03.000 --> 00:19:07.000
And that indicates that within the
serum, there is some kind of factor

00:19:07.000 --> 00:19:12.000
which is neutralizing the
infectivity of the polio virus

00:19:12.000 --> 00:19:16.000
particles, i.e. the poliovirus
particles are in some

00:19:16.000 --> 00:19:21.000
way prevented from creating an
infection, and their plaque forming

00:19:21.000 --> 00:19:26.000
ability, their ability corrode
these holes in the monolayers of

00:19:26.000 --> 00:19:31.000
subsequently infected
cultures is now compromised.

00:19:31.000 --> 00:19:35.000
And such activity suggests that such
an individual has actually antiserum,

00:19:35.000 --> 00:19:39.000
i.e. some kind of reactivity which
prevents the poliovirus particle

00:19:39.000 --> 00:19:44.000
from doing its thing. In
fact, I can tell you that such

00:19:44.000 --> 00:19:48.000
antiserum or such neutralizing
antiserum can be achieved in a

00:19:48.000 --> 00:19:53.000
number of ways. One way
to do it is just to take

00:19:53.000 --> 00:19:57.000
the protein capsid, the
outer coat of the poliovirus

00:19:57.000 --> 00:20:02.000
particle that I described before
and inject that into an individual.

00:20:02.000 --> 00:20:06.000
And that will on its own already
evoke a measure of antiviral

00:20:06.000 --> 00:20:10.000
immunity. It will evoke a
concentration of antiserum in the

00:20:10.000 --> 00:20:15.000
blood of a patient. Or, if
you want to do it in a very

00:20:15.000 --> 00:20:19.000
much more modern way, you can
sequence the proteins of the

00:20:19.000 --> 00:20:24.000
poliovirus and determine
the amino acid sequence.

00:20:24.000 --> 00:20:28.000
And when you do that, you
can synthesize through organic

00:20:28.000 --> 00:20:33.000
synthesis oligopeptides of
10 or 20 amino acids long.

00:20:33.000 --> 00:20:36.000
And these oligopeptides reflect
different parts of the poliovirus

00:20:36.000 --> 00:20:39.000
coat protein. So, if here's
the sequence of one of the

00:20:39.000 --> 00:20:43.000
capsid proteins, recall
that capsid refers to the

00:20:43.000 --> 00:20:46.000
coat that's shielding or
protecting in this case poliovirus,

00:20:46.000 --> 00:20:50.000
the single stranded viral RNA genome.
So, if here's one of the capsid

00:20:50.000 --> 00:20:53.000
proteins, what you can do is to
figure out the amino acid sequence

00:20:53.000 --> 00:20:57.000
of this segment of the capsid
protein, and then rather than

00:20:57.000 --> 00:21:00.000
cutting it out from the capsid
protein, you just synthesize it by

00:21:00.000 --> 00:21:04.000
organic synthesis, ten or
twenty amino acid residues

00:21:04.000 --> 00:21:08.000
long. And when
you inject that

00:21:08.000 --> 00:21:12.000
oligopeptides into an individual,
that may also evoke some kind of

00:21:12.000 --> 00:21:16.000
neutralizing antibody response.
In fact, as you might correctly

00:21:16.000 --> 00:21:21.000
imagine, if we can look in more
detail at the capsid structure,

00:21:21.000 --> 00:21:25.000
I'm just going to draw it
in a very primitive way here,

00:21:25.000 --> 00:21:29.000
it's obviously much more complicated.
But if this is the capsid here,

00:21:29.000 --> 00:21:34.000
it has an inside and an outside.
And the antibodies that you make

00:21:34.000 --> 00:21:39.000
against an oligopeptide that is
sticking out here on the outside

00:21:39.000 --> 00:21:43.000
will be neutralizing, i.e.
the antiserum will be able to

00:21:43.000 --> 00:21:48.000
recognize the outside of the virus
particle and somehow inactivate it.

00:21:48.000 --> 00:21:53.000
But if you were to use an
oligopeptide antigen from over here,

00:21:53.000 --> 00:21:57.000
which might be a part of the
capsid protein that is deep inside,

00:21:57.000 --> 00:22:02.000
well even though you might have a
good reactivity with oligopeptide it

00:22:02.000 --> 00:22:07.000
would not be neutralizing. And
that suggests the notion that

00:22:07.000 --> 00:22:11.000
whatever is present in the antiserum
is recognizing some external portion

00:22:11.000 --> 00:22:16.000
of this protein which is sticking
out. And that external portion is

00:22:16.000 --> 00:22:21.000
called an antigen. So,
there's a specific chemical

00:22:21.000 --> 00:22:25.000
structure which is being
recognized by the antiserum, and we

00:22:25.000 --> 00:22:30.000
call that an antigen. In
principal, there are dozens of

00:22:30.000 --> 00:22:34.000
distinct antigens on the
surface of a poliovirus particle,

00:22:34.000 --> 00:22:39.000
each of which one can, in principal,
make an immunizing antibody against.

00:22:39.000 --> 00:22:44.000
Each of those can therefore
be considered to be an antigen.

00:22:44.000 --> 00:22:48.000
And, how can we imagine this
antiserum as actually successfully

00:22:48.000 --> 00:22:53.000
neutralizing the virus particle?
And here, this depended on actually

00:22:53.000 --> 00:22:58.000
the biochemical characterization
of antisera because one soon came to

00:22:58.000 --> 00:23:03.000
learn that within antisera
are what we call antibodies.

00:23:03.000 --> 00:23:07.000
Or, keep in mind that in biology
you never should ever use a simple

00:23:07.000 --> 00:23:11.000
Anglo-Saxon word if you can use a
much more complicated Greek or Latin

00:23:11.000 --> 00:23:16.000
one. So, we can always
call these immunoglobulins.

00:23:16.000 --> 00:23:20.000
You see the word right there.
Immunoglobulin represents an

00:23:20.000 --> 00:23:25.000
antibody molecule, and it
turns out that much of the

00:23:25.000 --> 00:23:29.000
protein in the soluble fraction
of our serum consists of such

00:23:29.000 --> 00:23:34.000
antibody molecules. And these
antibody molecules have a

00:23:34.000 --> 00:23:38.000
very interesting structure which
I'd like to dwell upon momentarily.

00:23:38.000 --> 00:23:42.000
But before I do that, I'd just want
to anticipate what I'm about to say

00:23:42.000 --> 00:23:46.000
by indicating that we now realize
that an antibody molecule can

00:23:46.000 --> 00:23:50.000
recognize an antigen on the
surface of the virus particle,

00:23:50.000 --> 00:23:54.000
and the antibody molecule can
actually physically bind to this

00:23:54.000 --> 00:23:58.000
antigen. So, we have an
antigen antibody complex,

00:23:58.000 --> 00:24:02.000
and that binding of the antibody
molecule to the surface of the virus

00:24:02.000 --> 00:24:06.000
particle is what results
functionally in the neutralization

00:24:06.000 --> 00:24:11.000
of its infectivity. Or
to put it another way,

00:24:11.000 --> 00:24:15.000
given the fact that this particular
capsid protein might be repeated,

00:24:15.000 --> 00:24:19.000
let's say, 60 times around a
spherical surface of a virus

00:24:19.000 --> 00:24:23.000
particle, there might be, in
fact, 60 different antibody

00:24:23.000 --> 00:24:27.000
molecules recognizing this repeated
antigen which occurs on each of

00:24:27.000 --> 00:24:31.000
these capsid proteins.
And therefore,

00:24:31.000 --> 00:24:35.000
you can imagine in a very
approximate way that if you have an

00:24:35.000 --> 00:24:39.000
effective neutralizing antibody,
it's coating the surface of the

00:24:39.000 --> 00:24:42.000
virus particle of antibody molecules,
and that clearly obstructs the

00:24:42.000 --> 00:24:46.000
ability of the virus particle
to initiate a subsequent round of

00:24:46.000 --> 00:24:49.000
infection. Here's what a single
antibody molecule looks like,

00:24:49.000 --> 00:24:53.000
and it has some critically
important features. First of all,

00:24:53.000 --> 00:24:57.000
there is a specific region of
the antibody that recognizes

00:24:57.000 --> 00:25:01.000
the antigen. In this
case, I'm using as the

00:25:01.000 --> 00:25:05.000
example the antigen an oligopeptide
on the surface of the poliovirus

00:25:05.000 --> 00:25:09.000
particle. And that is located in
this part of the antibody molecule.

00:25:09.000 --> 00:25:14.000
Note, by the way, the antibody
molecule is a heterotetramer.

00:25:14.000 --> 00:25:18.000
It has two light chains, two
small chains here on the right,

00:25:18.000 --> 00:25:23.000
and two heavy chains. Note that
the cysteine disulphide bonds are

00:25:23.000 --> 00:25:27.000
holding the entire assembly together.
So here, one is not relying on

00:25:27.000 --> 00:25:32.000
hydrogen bonds to hold
together this heterotetramer.

00:25:32.000 --> 00:25:36.000
And note the following, that
there are portions of the

00:25:36.000 --> 00:25:41.000
antibody molecule which may
be specifically varied from one

00:25:41.000 --> 00:25:46.000
antibody molecule to another over
here, and other portions which are

00:25:46.000 --> 00:25:50.000
standard operating hardware. And
let me elaborate on that for a

00:25:50.000 --> 00:25:55.000
moment. If you look in the serum of
an individual and you try to analyze

00:25:55.000 --> 00:26:00.000
the chemical structure of the
antibody molecules in their serum,

00:26:00.000 --> 00:26:04.000
you find that for a given type
of antibody molecule like this,

00:26:04.000 --> 00:26:09.000
all the antibody molecules
share this constant here: light

00:26:09.000 --> 00:26:13.000
purple domain. They
all have that in common.

00:26:13.000 --> 00:26:17.000
They all have in common
also the light blue domains.

00:26:17.000 --> 00:26:21.000
But if you were able, and
we'll discuss this in a moment

00:26:21.000 --> 00:26:25.000
how you could do so, if you
were able to analyze this

00:26:25.000 --> 00:26:29.000
other portion, you would
find that when you look at

00:26:29.000 --> 00:26:33.000
one antibody molecule, it has
a certain amino acid sequence

00:26:33.000 --> 00:26:37.000
here, which is involved in antigen
recognition, and that this amino

00:26:37.000 --> 00:26:41.000
acid sequence on the light and heavy
chain varies from one heterotetramer

00:26:41.000 --> 00:26:45.000
to another heterotetramer. And
again, I'm going to reinforce

00:26:45.000 --> 00:26:50.000
that. But I want to indicate
that there are, within the human

00:26:50.000 --> 00:26:55.000
antiserum, millions of structurally
distinct antibody molecules.

00:26:55.000 --> 00:27:00.000
They share in common the structure
down here up to this point.

00:27:00.000 --> 00:27:03.000
And thereafter, each
of them has its own

00:27:03.000 --> 00:27:06.000
idiosyncratic,
its own unusual,

00:27:06.000 --> 00:27:09.000
its own particular private
antigen recognition site,

00:27:09.000 --> 00:27:13.000
which stems from a region or a
pocket of the protein which is

00:27:13.000 --> 00:27:16.000
involved in antigen recognition and
has variable amino acid sequences.

00:27:16.000 --> 00:27:19.000
Here, you see the way that an x-ray
crystallographer would actually

00:27:19.000 --> 00:27:23.000
visualize this molecule, and
you begin to realize that the

00:27:23.000 --> 00:27:26.000
schematic that I showed you before,
in fact, is really doing a little

00:27:26.000 --> 00:27:29.000
bit of violence to the
real amino acid sequence,

00:27:29.000 --> 00:27:33.000
to the real three dimensional
structure, excuse me.

00:27:33.000 --> 00:27:36.000
Now, note something else that is
implicit in what I've just said but

00:27:36.000 --> 00:27:40.000
I haven't said it explicitly,
there are actually two antigen

00:27:40.000 --> 00:27:44.000
recognition sites,
one here and one here.

00:27:44.000 --> 00:27:48.000
So, this antibody is in that
sense bivalent. And if there were,

00:27:48.000 --> 00:27:51.000
for example, two poliovirus
particles, one over here and one

00:27:51.000 --> 00:27:55.000
over here, you could imagine in
principal, although I'm not drawing

00:27:55.000 --> 00:27:59.000
things to scale, that this
antibody molecule could

00:27:59.000 --> 00:28:03.000
actually use one of its antigen
recognition sites to bind over here

00:28:03.000 --> 00:28:07.000
to one poliovirus particle,
and this to bind to another

00:28:07.000 --> 00:28:11.000
poliovirus particle. Now,
I've told you that the antigen

00:28:11.000 --> 00:28:15.000
is recognized by this antibody is
an oligopeptide, which constitutes a

00:28:15.000 --> 00:28:20.000
distinct chemical structure. How
many different oligopeptides of

00:28:20.000 --> 00:28:25.000
ten amino acids long are there
in the mathematical universe?

00:28:25.000 --> 00:28:30.000
How many conceivable
ones are there?

00:28:30.000 --> 00:28:34.000
How many amino acids are there?
20? So what's the number? I heard

00:28:34.000 --> 00:28:38.000
you all say 1020, right?
So, that's an awful lot.

00:28:38.000 --> 00:28:42.000
That's more than you can shake a
stick at. So that means that there

00:28:42.000 --> 00:28:46.000
are, in principal, essentially
an infinite number of

00:28:46.000 --> 00:28:50.000
oligopeptides of ten amino acids
long, and each one of those could,

00:28:50.000 --> 00:28:54.000
in principal constitute a distinct
antigen that could be recognized by

00:28:54.000 --> 00:28:58.000
the antibody molecule. Note
that what's happening here is

00:28:58.000 --> 00:29:02.000
that a protein antigen such as
one of these oligopeptides is being

00:29:02.000 --> 00:29:06.000
recognized by another protein,
which is the antibody molecule.

00:29:06.000 --> 00:29:10.000
And therefore, we begin
to imagine there's some

00:29:10.000 --> 00:29:14.000
kind of lock and key complementarity
where somehow the amino acid

00:29:14.000 --> 00:29:18.000
sequence here recognizes and binds a
complementary fashion to the antigen

00:29:18.000 --> 00:29:22.000
that is being recognized, and
there's a physical association

00:29:22.000 --> 00:29:26.000
which allows the antibody molecule
then to attach tightly to the

00:29:26.000 --> 00:29:30.000
antigen bearing
poliovirus particle.

00:29:30.000 --> 00:29:34.000
And, by the way, to
put it another way,

00:29:34.000 --> 00:29:39.000
it could be that some of these
poliovirus capsid proteins have not

00:29:39.000 --> 00:29:43.000
yet been assembled in a virus
particle. And even if they aren't,

00:29:43.000 --> 00:29:48.000
in principal a free protein,
a capsid protein of poliovirus,

00:29:48.000 --> 00:29:53.000
could also be recognized by an
antibody molecule and be bound by

00:29:53.000 --> 00:29:57.000
the antibody molecule. Now,
this creates several major

00:29:57.000 --> 00:30:02.000
conceptual problems. How
on earth can the body make

00:30:02.000 --> 00:30:06.000
antibody molecules that are able
to recognize a poliovirus particle?

00:30:06.000 --> 00:30:10.000
Well, you'll say that's easy.
Clearly, in our genes there must be

00:30:10.000 --> 00:30:14.000
some kind of nucleotide sequence
which has up here the ability encode

00:30:14.000 --> 00:30:19.000
an antigen-binding site.
Note, by the way, the

00:30:19.000 --> 00:30:23.000
antigen-binding site's actually
composed of two different proteins.

00:30:23.000 --> 00:30:27.000
Here's part of the antigen-binding
site. Here's another part of the

00:30:27.000 --> 00:30:31.000
antigen-binding site. So,
it must be on two different

00:30:31.000 --> 00:30:35.000
genes. But, these two different
genes create an antigen-binding site

00:30:35.000 --> 00:30:39.000
which can recognize poliovirus.
Bind the poliovirus antigen and

00:30:39.000 --> 00:30:43.000
neutralize it. Well,
that's well and good.

00:30:43.000 --> 00:30:47.000
That's perfectly reasonable,
but let me tell you what's

00:30:47.000 --> 00:30:51.000
unreasonable about that.
Each of us during his or her

00:30:51.000 --> 00:30:55.000
lifetime are going to be infected
by literally hundreds of viruses.

00:30:55.000 --> 00:30:59.000
You get cold viruses almost
every two, three, four, five

00:30:59.000 --> 00:31:03.000
times a year. By
the time you get old,

00:31:03.000 --> 00:31:07.000
you actually have acquired
immunity to many of these,

00:31:07.000 --> 00:31:11.000
but during the course of one's
lifetime, the immune system has been

00:31:11.000 --> 00:31:15.000
able to generate antiviral immunity
and eliminate virtually all the

00:31:15.000 --> 00:31:19.000
viral infections you ever got.
How many people do you know who

00:31:19.000 --> 00:31:23.000
have actually died of a viral
infection? And yet each one of

00:31:23.000 --> 00:31:27.000
these viruses is in principal able
to replicate throughout your body

00:31:27.000 --> 00:31:32.000
and kill you, and they never do?
So that means that the immune system

00:31:32.000 --> 00:31:36.000
is extremely effective in developing
an antibody or antibodies which

00:31:36.000 --> 00:31:40.000
could neutralize infecting virus
particles. You might have the cold

00:31:40.000 --> 00:31:44.000
for a day, a week, or two
weeks, but eventually the

00:31:44.000 --> 00:31:48.000
immune system develops enough
antibodies and other strategies to

00:31:48.000 --> 00:31:52.000
eliminate the viral infections
from your body. I remember when my

00:31:52.000 --> 00:31:56.000
grandfather was 94 years old and
he got a cold. And he hadn't had a

00:31:56.000 --> 00:32:00.000
cold for 20 years because by the age
of 75, he had been exposed to almost

00:32:00.000 --> 00:32:04.000
every virus imaginable. And
so, when he got a cold at the

00:32:04.000 --> 00:32:08.000
age of 94, his brain was still
working, he'd forgotten how to blow

00:32:08.000 --> 00:32:12.000
his nose. It just hadn't happened
to him. He didn't know what it was

00:32:12.000 --> 00:32:16.000
like to have a cold anymore.
So, it just goes to show you that

00:32:16.000 --> 00:32:20.000
the immune system can be very
versatile and very successful.

00:32:20.000 --> 00:32:24.000
But this creates a major conceptual
problem. How do our genes know how

00:32:24.000 --> 00:32:28.000
to create antigen-binding sites in
our antibodies that recognize all of

00:32:28.000 --> 00:32:32.000
the infectious viruses that
are going to attack us during

00:32:32.000 --> 00:32:36.000
a lifetime. Do we have
a gene for neutralizing

00:32:36.000 --> 00:32:40.000
smallpox? Do we have a gene
for neutralizing poliovirus and

00:32:40.000 --> 00:32:44.000
adenovirus, which gives us bad
head colds. And, rabies virus and

00:32:44.000 --> 00:32:48.000
vesicular stomatitis virus, and
all other kinds of viruses that

00:32:48.000 --> 00:32:52.000
we are going to experience
in a lifetime. Well,

00:32:52.000 --> 00:32:56.000
possibly we do have a gene
for each one of those. But,

00:32:56.000 --> 00:33:00.000
then I'm beginning to raise some
other questions if you believe that,

00:33:00.000 --> 00:33:04.000
which you shouldn't. First
of all, there are many

00:33:04.000 --> 00:33:08.000
different distinct oligopeptide
antigens on the surface of the

00:33:08.000 --> 00:33:12.000
poliovirus particle. If you
look at the antibodies that

00:33:12.000 --> 00:33:16.000
had been developed against a
poliovirus capsid protein in an

00:33:16.000 --> 00:33:19.000
immunized individual,
they have one antibody that

00:33:19.000 --> 00:33:23.000
recognizes one part of these capsid
protein, and another antibody that

00:33:23.000 --> 00:33:27.000
recognized another part.
So, here's the surface of the

00:33:27.000 --> 00:33:31.000
capsid protein blown up now,
and we can imagine there are

00:33:31.000 --> 00:33:35.000
different oligopeptide epitopes
on different parts of the protein.

00:33:35.000 --> 00:33:39.000
And they're all ten amino acids
long, for example. The fact is,

00:33:39.000 --> 00:33:43.000
in individuals who have
anti-poliovirus antibody,

00:33:43.000 --> 00:33:48.000
you can find that there's an
antibody that recognizes this

00:33:48.000 --> 00:33:52.000
antigen, and one that recognizes
this antigen, and another that

00:33:52.000 --> 00:33:57.000
recognizes this antigen. So,
the number of distinct antigens

00:33:57.000 --> 00:34:01.000
which the immune system has
succeeded in making antibodies

00:34:01.000 --> 00:34:05.000
against, vastly exceeds the number
of infectious agents we're going to

00:34:05.000 --> 00:34:10.000
experience in a lifetime. And,
that begins to undermine your

00:34:10.000 --> 00:34:14.000
confidence in the notion that when
we're born, we already possess the

00:34:14.000 --> 00:34:18.000
genes to recognize
different infectious agents,

00:34:18.000 --> 00:34:22.000
and to construct neutralizing
antibodies against them.

00:34:22.000 --> 00:34:26.000
There's also another flaw in this
whole notion that we inherit a set

00:34:26.000 --> 00:34:30.000
of genes that enable us from
the get-go to make specific

00:34:30.000 --> 00:34:34.000
antibodies. And that
flaw comes from the

00:34:34.000 --> 00:34:38.000
following notion. Evolution
cannot anticipate which

00:34:38.000 --> 00:34:42.000
infectious agents each one of us is
going to experience in a lifetime.

00:34:42.000 --> 00:34:47.000
Let's say that some of us
experience a totally new kind of a

00:34:47.000 --> 00:34:51.000
viral infection which had never
been experienced by our ancestors.

00:34:51.000 --> 00:34:55.000
If the way of neutralizing that
virus, and defending us against that

00:34:55.000 --> 00:35:00.000
infection depended on using an
antibody molecule whose sequence was

00:35:00.000 --> 00:35:04.000
encoded in our germ line in
the genes we inherited at birth,

00:35:04.000 --> 00:35:08.000
then we might be in bad shape if
that virus was a novel virus to

00:35:08.000 --> 00:35:13.000
which the human race
had never been exposed.

00:35:13.000 --> 00:35:17.000
And in arguing that, I'm
telling you that it's impossible

00:35:17.000 --> 00:35:21.000
to imagine a situation where our
germ line, the set of genes we are

00:35:21.000 --> 00:35:26.000
born with that we start life with,
already contains the information for

00:35:26.000 --> 00:35:30.000
making each one of the different
kinds of antibodies that

00:35:30.000 --> 00:35:34.000
are in our antiserum. How
many different kinds of

00:35:34.000 --> 00:35:38.000
antibodies are in our antiserum?
Well now, the whole notion becomes

00:35:38.000 --> 00:35:42.000
even more stark because
there's probably millions,

00:35:42.000 --> 00:35:46.000
maybe even ten or 100 million
distinct antibody molecules floating

00:35:46.000 --> 00:35:50.000
around in our blood. Each
one in this case has the same

00:35:50.000 --> 00:35:54.000
constant region down here.
You see the constant region,

00:35:54.000 --> 00:35:58.000
and each one of these 100 million
distinct antibody species has its

00:35:58.000 --> 00:36:01.000
own antigen-combining site.
Now, if we want to pursue that

00:36:01.000 --> 00:36:05.000
further, we have to begin to imagine
how the immune system can make so

00:36:05.000 --> 00:36:09.000
many different antibody molecules.
This itself is a major conceptual

00:36:09.000 --> 00:36:13.000
challenge. And now we have to
go and know a little bit of cell

00:36:13.000 --> 00:36:16.000
biology because if we
look at the immune system,

00:36:16.000 --> 00:36:20.000
and there are cells forming
in the immune system,

00:36:20.000 --> 00:36:24.000
we find a set of cells that are
called B cells or a subset of B

00:36:24.000 --> 00:36:28.000
cells that are called plasma cells.
And the plasma cells, and there's a

00:36:28.000 --> 00:36:32.000
whole bunch of
different plasma cells.

00:36:32.000 --> 00:36:35.000
So the plasma cells are floating
around in the circulation,

00:36:35.000 --> 00:36:39.000
and the plasma cells are able to
secrete antibody molecules into the

00:36:39.000 --> 00:36:43.000
serum. And they are abundant.
They're around by the billions

00:36:43.000 --> 00:36:47.000
inside our body. And
these plasma cells are

00:36:47.000 --> 00:36:51.000
constantly secreting antibody
molecules into the serum around them

00:36:51.000 --> 00:36:55.000
into the plasma around them.
And these are the antibody

00:36:55.000 --> 00:36:59.000
molecules I've been talking about
here, the antibody molecules that

00:36:59.000 --> 00:37:03.000
are capable of neutralizing
poliovirus particles.

00:37:03.000 --> 00:37:06.000
So, this now creates
another conceptual question.

00:37:06.000 --> 00:37:10.000
Let's imagine for the sake of
argument that in our blood stream

00:37:10.000 --> 00:37:13.000
there are 100 million distinct
antigen species floating around.

00:37:13.000 --> 00:37:17.000
And I don't mean 100 million
molecules. I mean there's a million

00:37:17.000 --> 00:37:21.000
molecules, and each one of
these species has a different

00:37:21.000 --> 00:37:24.000
antigen-binding site.
So, there may be many

00:37:24.000 --> 00:37:28.000
antigen-binding sites.
There may be many antibody

00:37:28.000 --> 00:37:32.000
molecules. There may be
million of this kind of

00:37:32.000 --> 00:37:36.000
antibody that recognize this
antigen. There may be millions of this

00:37:36.000 --> 00:37:40.000
antibody recognizing this
antigen, millions of this antibody

00:37:40.000 --> 00:37:44.000
recognizing this antigen on
the surface of poliovirus.

00:37:44.000 --> 00:37:48.000
And I'm saying there could be
a million distinct species of

00:37:48.000 --> 00:37:52.000
antibodies, each species being
defined by the antigen that

00:37:52.000 --> 00:37:56.000
recognizes and by the structure
of its antigen-recognizing site,

00:37:56.000 --> 00:38:00.000
its antigen-binding
site. In other words,

00:38:00.000 --> 00:38:03.000
to repeat myself, what
distinguishes one species of

00:38:03.000 --> 00:38:06.000
antibodies from another
is the amino acid sequence,

00:38:06.000 --> 00:38:09.000
the structure of this particular
region of the antibody molecule that

00:38:09.000 --> 00:38:12.000
makes one species of antibody
different from the other.

00:38:12.000 --> 00:38:16.000
And with all that in mind, we
have two alternative scenarios.

00:38:16.000 --> 00:38:19.000
Let's again say there's a million
different species of antibodies

00:38:19.000 --> 00:38:22.000
being secreted into the antiserum
at any one point in time.

00:38:22.000 --> 00:38:25.000
This cell could make all million.
This cell could make the whole

00:38:25.000 --> 00:38:28.000
million species.
This cell could,

00:38:28.000 --> 00:38:32.000
and this cell could.
In other words,

00:38:32.000 --> 00:38:38.000
each one of these B cells
could be multitalented,

00:38:38.000 --> 00:38:43.000
able to simultaneously make a
million different kinds of antibody

00:38:43.000 --> 00:38:49.000
molecules. The opposite model is
as follows, and that is that this B

00:38:49.000 --> 00:38:54.000
cell makes one species of antibody.
We'll call it antibody A, and this

00:38:54.000 --> 00:39:00.000
species makes another
species of antibody molecule.

00:39:00.000 --> 00:39:04.000
So, this B cell makes antibody A.
This B cell makes antibody B. This

00:39:04.000 --> 00:39:08.000
B cell makes antibody C. And
keep in mind that when I'm

00:39:08.000 --> 00:39:12.000
saying antibody A, B, and
C, I mean A has a certain

00:39:12.000 --> 00:39:17.000
antigen-binding site. Be has
another antigen binding site.

00:39:17.000 --> 00:39:21.000
C has yet another antigen binding
site. And, we can distinguish

00:39:21.000 --> 00:39:25.000
between these two alternative
mechanistic models by looking at the

00:39:25.000 --> 00:39:29.000
disease called multiple myeloma.
And what you have in multiple

00:39:29.000 --> 00:39:33.000
myeloma is the following.
All of a sudden,

00:39:33.000 --> 00:39:37.000
the blood stream becomes full of
much greatly elevated levels of

00:39:37.000 --> 00:39:41.000
antibody molecules. They're
present in the blood stream.

00:39:41.000 --> 00:39:45.000
Now, normally, if you
look at all the antibody

00:39:45.000 --> 00:39:48.000
molecules in the blood stream and
you were to separate them by some

00:39:48.000 --> 00:39:52.000
kind of electrophoretic technique
which distinguished them on the

00:39:52.000 --> 00:39:56.000
basis of subtle differences
in the amino acid sequence,

00:39:56.000 --> 00:40:00.000
and let's not worry about
exactly how you do that.

00:40:00.000 --> 00:40:03.000
But we've talked about a million
different antibody species.

00:40:03.000 --> 00:40:07.000
They're chemically slightly
different from one another by virtue

00:40:07.000 --> 00:40:11.000
of the antigen-recognizing site.
And if you could separate them by

00:40:11.000 --> 00:40:15.000
some kind of system that separated
them on the basis of their charge,

00:40:15.000 --> 00:40:18.000
you'd find a whole spectrum of
different antibody molecules,

00:40:18.000 --> 00:40:22.000
a million in this large,
heterogeneous mixture of antibody

00:40:22.000 --> 00:40:26.000
molecules, each species found
somewhere or another in this very

00:40:26.000 --> 00:40:30.000
broad distribution of 100 million
distinct species which are all mixed

00:40:30.000 --> 00:40:33.000
together in the antiserum. If
you look at the serum or the

00:40:33.000 --> 00:40:37.000
plasma of a patient suffering
from multiple myeloma,

00:40:37.000 --> 00:40:41.000
what you find is the following,
that one species of antibody

00:40:41.000 --> 00:40:45.000
molecules dominates over all
the other ones. And so now,

00:40:45.000 --> 00:40:49.000
whereas that single species might
only be on average one millionth of

00:40:49.000 --> 00:40:53.000
the total antibody complement in
the blood. In those suffering from

00:40:53.000 --> 00:40:57.000
myeloma, or sometimes it's
called multiple myeloma,

00:40:57.000 --> 00:41:01.000
now one single antibody
species may represent 60, 70,

00:41:01.000 --> 00:41:05.000
or 80% of all the antibody
molecules in the blood.

00:41:05.000 --> 00:41:09.000
And clearly something has gone
very wrong. And if you look at what

00:41:09.000 --> 00:41:14.000
happened in the case of an
individual of multiple myeloma,

00:41:14.000 --> 00:41:18.000
if we look at that diagram up there,
what you see now is that instead of

00:41:18.000 --> 00:41:23.000
there being many different B cells
that are equivalently represented in

00:41:23.000 --> 00:41:28.000
the blood stream, now one
of the B cell types has

00:41:28.000 --> 00:41:32.000
begun to multiple uncontrollably,
and this vasicular B cell type,

00:41:32.000 --> 00:41:37.000
which I indicate up there,
which happens to be specifically

00:41:37.000 --> 00:41:42.000
able to make antibody C in this
model, now this particular species

00:41:42.000 --> 00:41:46.000
of B cells is now a predominant
constituent of the population of B

00:41:46.000 --> 00:41:50.000
cells in the blood.
So just to reiterate,

00:41:50.000 --> 00:41:54.000
in the normal blood, there are a
million different sub-populations of

00:41:54.000 --> 00:41:58.000
B cells. But in these individuals,
one of the sub-populations of B

00:41:58.000 --> 00:42:01.000
cells has expanded. It's
created a monoclonal growth.

00:42:01.000 --> 00:42:05.000
Keep in mind monoclonal refers to
the fact that all the cells in this

00:42:05.000 --> 00:42:09.000
tumor, and there is a tumor,
descend from the same ancestor.

00:42:09.000 --> 00:42:13.000
So, it's a monoclonal
growth. It's a kind of cancer.

00:42:13.000 --> 00:42:17.000
It's a kind of blood cancer.
And all the cells in this

00:42:17.000 --> 00:42:21.000
population make the
identical antibody molecule,

00:42:21.000 --> 00:42:25.000
which explains this very homogeneous
peak here riding above the very

00:42:25.000 --> 00:42:29.000
heterogeneous mixture of antibodies,
the background heterogeneity that

00:42:29.000 --> 00:42:33.000
normally characterizes
normal serum.

00:42:33.000 --> 00:42:36.000
And what that clearly indicates is
that each different B cell or plasma

00:42:36.000 --> 00:42:40.000
cell as you will like to call it in
our normal serum is specialized to

00:42:40.000 --> 00:42:44.000
make its own particular kind of
antibody. It's not that a single B

00:42:44.000 --> 00:42:48.000
cell can make a million different
antibodies. Each B cell makes a

00:42:48.000 --> 00:42:52.000
very specific kind of antibody.
And when I say a specific antibody

00:42:52.000 --> 00:42:56.000
or a specific antibody species,
again I'm referring to an antibody

00:42:56.000 --> 00:43:00.000
that has a specific
antigen-binding site.

00:43:00.000 --> 00:43:04.000
Or to put it another way, all
of the antibody molecules coming

00:43:04.000 --> 00:43:09.000
out of a single B cell are
identical with one another.

00:43:09.000 --> 00:43:14.000
That, then, raises the question of
how the B cell learns how to make

00:43:14.000 --> 00:43:19.000
that antibody molecule and
not other antibody molecules.

00:43:19.000 --> 00:43:23.000
There's yet another puzzle we have
to answer, and that's the following.

00:43:23.000 --> 00:43:28.000
If you look at the serum response
of an individual to a viral

00:43:28.000 --> 00:43:32.000
infection, it looks like this.
And here, let's look at what's

00:43:32.000 --> 00:43:36.000
present in the ordinate, and
it's on a log scale as you can

00:43:36.000 --> 00:43:40.000
see here, and what's present on
the abscissa. If one is initially

00:43:40.000 --> 00:43:44.000
exposed to an antigen and develops
an immunity, then here is the wave

00:43:44.000 --> 00:43:48.000
of antibody production and the
concentration of antibody against,

00:43:48.000 --> 00:43:52.000
let's say, poliovirus that is
present in the blood of a recently

00:43:52.000 --> 00:43:56.000
infected individual. They're
exposed to the antigen,

00:43:56.000 --> 00:44:00.000
or in this case the
poliovirus capsid protein.

00:44:00.000 --> 00:44:04.000
In the days and weeks that follow,
they develop a significant level of

00:44:04.000 --> 00:44:09.000
antibody which is used to neutralize,
in this case the infected poliovirus

00:44:09.000 --> 00:44:13.000
particles. And then once the
poliovirus is cleared from the

00:44:13.000 --> 00:44:18.000
system, i.e. once all the infectious
particles have been neutralized,

00:44:18.000 --> 00:44:23.000
then the antibody molecules
go down almost to zero,

00:44:23.000 --> 00:44:28.000
and there's virtually no antibody
against poliovirus left around.

00:44:28.000 --> 00:44:32.000
But look what happens when
that individual is re-exposed to

00:44:32.000 --> 00:44:37.000
poliovirus years later. All
of a sudden, now he or she

00:44:37.000 --> 00:44:41.000
mounts a massive antibody response
vastly higher than the initial one.

00:44:41.000 --> 00:44:46.000
Well, you say it's only
twofold higher than this.

00:44:46.000 --> 00:44:50.000
But let's look at the ordinate.
This is a logarithmic plot. So,

00:44:50.000 --> 00:44:55.000
on this plot, the secondary antibody
response is maybe 100 or 200 times

00:44:55.000 --> 00:45:00.000
more vigorous, much
higher antibody production.

00:45:00.000 --> 00:45:04.000
Let's think about this time years
later when that person is exposed to

00:45:04.000 --> 00:45:08.000
a second virus. Let's
say that person is exposed

00:45:08.000 --> 00:45:13.000
subsequently to cold virus,
adenovirus. Does the exposure to

00:45:13.000 --> 00:45:17.000
poliovirus here in the blue
curve help that person develop an

00:45:17.000 --> 00:45:21.000
adenovirus antibody response?
The answer is absolutely not.

00:45:21.000 --> 00:45:26.000
There's no cross-immunity. And so,
when that individual years later

00:45:26.000 --> 00:45:30.000
becomes exposed to adenovirus,
there's once again the same kind of

00:45:30.000 --> 00:45:34.000
initial response that happened as
a consequence of previously being

00:45:34.000 --> 00:45:39.000
exposed to poliovirus.
It's a rather weak response.

00:45:39.000 --> 00:45:43.000
It's effective enough in developing
the initial virus eliminating

00:45:43.000 --> 00:45:47.000
ability, but it's not very strong.
What is this telling us? I'm glad

00:45:47.000 --> 00:45:51.000
I asked that question. It's
telling us that that immune

00:45:51.000 --> 00:45:55.000
system is able to remember
over a period of months,

00:45:55.000 --> 00:45:59.000
years, and decades that a previous
exposure to this antigen has

00:45:59.000 --> 00:46:03.000
occurred because here the immune
system in this second red curve

00:46:03.000 --> 00:46:07.000
knows, somehow remembers, that
there was an exposure years

00:46:07.000 --> 00:46:12.000
earlier. And not only
does it remember and

00:46:12.000 --> 00:46:16.000
respond rapidly, but it
responds much more vigorously.

00:46:16.000 --> 00:46:20.000
And here now, we begin
to recognize some of the

00:46:20.000 --> 00:46:24.000
outlines of immunity because the
immune system remembers this earlier

00:46:24.000 --> 00:46:28.000
exposure, and when it's provoked a
second time, through an accidental

00:46:28.000 --> 00:46:32.000
and inadvertent exposure to
poliovirus on a second occasion,

00:46:32.000 --> 00:46:36.000
now it really goes to town. And
now it creates antisera which is

00:46:36.000 --> 00:46:40.000
much more vigorous than it
was during the first exposure.

00:46:40.000 --> 00:46:44.000
If the first exposure had never
occurred, there wouldn't be such a

00:46:44.000 --> 00:46:47.000
vigorous response. Look
at this one over here.

00:46:47.000 --> 00:46:51.000
So now, we begin to realize two
mysteries that we have to explain

00:46:51.000 --> 00:46:55.000
next time in our further
discussion of the immune system.

00:46:55.000 --> 00:46:59.000
First of all, how do B cells figure
out how to make so many different

00:46:59.000 --> 00:47:03.000
kinds of antibody molecules.
And secondly, how does the immune

00:47:03.000 --> 00:47:07.000
system remember from one decade
to the next, that exposure to a

00:47:07.000 --> 00:47:12.000
previous antigen has occurred
which merits a vigorous response.

00:47:12.000 --> 00:47:17.000
See you then
on Friday.