WEBVTT 00:00:16.492 --> 00:00:17.950 ADAM MARTIN: And today, we're going 00:00:17.950 --> 00:00:23.250 to talk about immunity, which is important, especially 00:00:23.250 --> 00:00:26.280 at this time of the year. 00:00:26.280 --> 00:00:39.210 So immunity is the resistance to disease 00:00:39.210 --> 00:00:41.310 based on a prior exposure. 00:00:44.640 --> 00:00:48.400 Based on prior exposure. 00:00:51.870 --> 00:00:57.100 And of course, this is the principle behind vaccination. 00:00:57.100 --> 00:01:01.170 So humans have been sort of using 00:01:01.170 --> 00:01:03.690 the properties of the immune system 00:01:03.690 --> 00:01:08.970 to prevent themselves from getting disease for centuries. 00:01:08.970 --> 00:01:13.170 One of the first very clear examples of this 00:01:13.170 --> 00:01:17.550 is back in the 18th century with the English physician, Edward 00:01:17.550 --> 00:01:25.440 Jenner, and Edward Jenner found out or came to the realization 00:01:25.440 --> 00:01:30.210 that farm hands on farms, specifically 00:01:30.210 --> 00:01:36.690 milk maids, that were exposed to a variant of smallpox 00:01:36.690 --> 00:01:39.630 from cows, which is known as cow pox, 00:01:39.630 --> 00:01:42.510 could become immune to smallpox. 00:01:42.510 --> 00:01:48.450 So cow pox is a less severe form of the disease. 00:01:48.450 --> 00:01:53.100 And what Jenner did was to take pustules 00:01:53.100 --> 00:01:58.710 from individuals who had the cow pox disease 00:01:58.710 --> 00:02:02.340 and inject them into an eight-year-old boy, 00:02:02.340 --> 00:02:04.950 and then infect that boy with smallpox 00:02:04.950 --> 00:02:11.430 to show that the boy was immune to smallpox after having 00:02:11.430 --> 00:02:15.210 received the cow pox material. 00:02:15.210 --> 00:02:17.670 And so this is the first example of the vaccine. 00:02:20.570 --> 00:02:25.740 And because the vaccine was derived from basically someone 00:02:25.740 --> 00:02:28.500 with cow pox, the word vaccine is 00:02:28.500 --> 00:02:32.610 from the Latin root of vaca, which is cow, 00:02:32.610 --> 00:02:36.840 so that's where the word vaccine comes from, OK? 00:02:36.840 --> 00:02:39.840 So today, we're going to talk about the systems 00:02:39.840 --> 00:02:45.660 that our bodies have to fight disease 00:02:45.660 --> 00:02:49.200 and there are several different all levels 00:02:49.200 --> 00:02:51.970 of the immune system. 00:02:51.970 --> 00:02:54.460 So I'm going to talk about two of them. 00:02:54.460 --> 00:02:59.390 So I'll talk about two levels of immunity. 00:03:05.290 --> 00:03:08.080 The first I want to mention is the one 00:03:08.080 --> 00:03:10.940 that we're all just like born with, 00:03:10.940 --> 00:03:12.670 which is known as innate immunity. 00:03:16.037 --> 00:03:16.999 [SNEEZING] 00:03:18.923 --> 00:03:20.380 [SNEEZING] 00:03:20.380 --> 00:03:23.140 Bless you. 00:03:23.140 --> 00:03:27.170 So innate immunity, as the name implies, 00:03:27.170 --> 00:03:30.295 is something that we are born with, so this is inborn. 00:03:37.440 --> 00:03:42.300 It also doesn't have a delay in when it's activated, right? 00:03:42.300 --> 00:03:45.330 So if you have an infection, this 00:03:45.330 --> 00:03:47.750 is sort of the first line of defense, right? 00:03:47.750 --> 00:03:53.310 There is an immediate response, and that's the innate immune 00:03:53.310 --> 00:03:55.155 system, so this is immediate. 00:03:55.155 --> 00:03:57.060 Here, I'll put this down here. 00:03:57.060 --> 00:03:57.900 It's immediate. 00:04:03.260 --> 00:04:09.180 So one example is of an innate immune response. 00:04:09.180 --> 00:04:11.810 This is not in the body but ex vivo, 00:04:11.810 --> 00:04:14.330 but here you see a human neutrophil, 00:04:14.330 --> 00:04:17.690 and neutrophils are part of our innate immune system. 00:04:17.690 --> 00:04:21.260 And neutrophils hunt and kill bacteria, right? 00:04:21.260 --> 00:04:25.820 You see that neutrophil chasing after that bacterium, 00:04:25.820 --> 00:04:27.080 and it's going for it. 00:04:27.080 --> 00:04:29.780 It's really trying to get it, but that bacteria really 00:04:29.780 --> 00:04:31.750 wants to get away, but got it! 00:04:31.750 --> 00:04:32.720 OK, great. 00:04:32.720 --> 00:04:37.380 So these neutrophils are part of the innate immune system. 00:04:37.380 --> 00:04:39.680 It's inborn, it's immediate. 00:04:39.680 --> 00:04:44.060 And in addition, the response of the innate immune system 00:04:44.060 --> 00:04:47.780 doesn't really change if you've been 00:04:47.780 --> 00:04:55.760 exposed to an infectious agent prior to the-- 00:04:55.760 --> 00:05:02.450 OK, so this does not change. 00:05:02.450 --> 00:05:05.120 I'm using the Greek delta for change. 00:05:05.120 --> 00:05:11.580 Does not change with prior exposure. 00:05:11.580 --> 00:05:14.900 OK, so it's sort of like a constant surveillance 00:05:14.900 --> 00:05:20.180 mechanism in your body that will go after foreign agents, OK? 00:05:20.180 --> 00:05:23.150 Now, this is very different from the next level 00:05:23.150 --> 00:05:28.580 of immunity, which is known as adaptive immunity. 00:05:36.350 --> 00:05:40.610 And as the name adaptive immunity implies, 00:05:40.610 --> 00:05:43.640 this is a type of immunity that does change. 00:05:43.640 --> 00:05:45.920 It adapts, OK? 00:05:45.920 --> 00:05:50.250 And this type of immunity is acquired, 00:05:50.250 --> 00:05:52.700 so it's also known as acquired immunity, 00:05:52.700 --> 00:06:01.970 but it's acquired with exposure to a foreign agent, OK? 00:06:01.970 --> 00:06:07.430 So this involves a change in immunity, this one does not, 00:06:07.430 --> 00:06:10.310 but the innate immune response is immediate, 00:06:10.310 --> 00:06:13.490 whereas adaptive immunity takes time. 00:06:13.490 --> 00:06:15.485 There's a delay, so this is also delayed. 00:06:23.870 --> 00:06:27.560 It's also highly specific, OK? 00:06:27.560 --> 00:06:30.920 So it's highly specific to the foreign agents 00:06:30.920 --> 00:06:34.610 that you are infected with. 00:06:34.610 --> 00:06:37.300 The innate immune system is less specific. 00:06:37.300 --> 00:06:40.190 It'll recognize, like, things like bacteria, 00:06:40.190 --> 00:06:43.280 but it won't be able to necessarily distinguish 00:06:43.280 --> 00:06:46.890 between different types of bacteria. 00:06:46.890 --> 00:06:50.520 So this is more specific than the innate immune system. 00:06:50.520 --> 00:06:53.750 This is why every year, you have to get a flu shot, 00:06:53.750 --> 00:06:56.660 because the flu virus is constantly changing. 00:06:56.660 --> 00:06:59.150 And our immune system is so specific 00:06:59.150 --> 00:07:02.060 that unless we get a new vaccination, 00:07:02.060 --> 00:07:07.280 our bodies will not be able to recognize it, OK? 00:07:07.280 --> 00:07:12.230 So this is-- so now I'm going to break down adaptive immunity 00:07:12.230 --> 00:07:13.175 into two branches. 00:07:17.310 --> 00:07:26.510 One is known as humoral immunity, 00:07:26.510 --> 00:07:40.370 and humoral immunity is basically protein-mediated, 00:07:40.370 --> 00:07:42.020 and there are proteins that mediate 00:07:42.020 --> 00:07:44.480 this are called antibodies. 00:07:49.350 --> 00:07:59.350 These antibodies are proteins, and it's called humoral 00:07:59.350 --> 00:08:02.080 because the antibodies can be secreted 00:08:02.080 --> 00:08:05.860 into the fluids or humors of our body, which is basically 00:08:05.860 --> 00:08:07.870 the blood, OK? 00:08:07.870 --> 00:08:09.790 So there is humoral immunity. 00:08:09.790 --> 00:08:12.160 The other type of adaptive immunity 00:08:12.160 --> 00:08:20.100 is cell mediated, and one thing I 00:08:20.100 --> 00:08:24.960 want to point out that the types of cells that make an antibody 00:08:24.960 --> 00:08:25.965 are known as B cells. 00:08:29.520 --> 00:08:32.250 What the B stands for isn't really important, 00:08:32.250 --> 00:08:34.980 but one thing that's helpful is that these cells 00:08:34.980 --> 00:08:44.670 mature in the bone marrow, and B stands for bone marrow, OK? 00:08:44.670 --> 00:08:46.590 So you can always remember where they mature. 00:08:49.800 --> 00:08:53.650 Now cell-mediated immunity, in contrast, 00:08:53.650 --> 00:08:57.780 involves a different type of cell called a T cell, 00:08:57.780 --> 00:09:00.690 and the T of T cell stands for thymus 00:09:00.690 --> 00:09:02.625 because these cells mature in the thymus. 00:09:11.760 --> 00:09:17.410 And I just want to point out where these cells come from. 00:09:17.410 --> 00:09:21.970 So we talked about adult stem cells earlier, 00:09:21.970 --> 00:09:26.590 and in this case, these T and B lymphocytes over here 00:09:26.590 --> 00:09:30.340 are derived from a multipotent hematopoietic stem 00:09:30.340 --> 00:09:33.970 cell, which generates a whole bunch 00:09:33.970 --> 00:09:35.900 of different types of cells. 00:09:35.900 --> 00:09:39.580 Many of them are involved in the innate immune response, 00:09:39.580 --> 00:09:43.450 but this common lymphoid progenitor over here 00:09:43.450 --> 00:09:45.040 gives rise to D-- 00:09:45.040 --> 00:09:47.890 T lymphocytes and B lymphocytes, which are 00:09:47.890 --> 00:09:52.060 involved in adaptive immunity. 00:09:52.060 --> 00:09:54.960 OK, so it's not important that you remember where-- 00:09:54.960 --> 00:09:58.690 what all these cells come from or what they, like, 00:09:58.690 --> 00:10:03.220 what the tree is, but that these cells arise 00:10:03.220 --> 00:10:07.030 from a common progenitor cell. 00:10:07.030 --> 00:10:14.080 OK, so both of these branches of the adaptive immune system 00:10:14.080 --> 00:10:19.260 have what are known as antigen receptors. 00:10:19.260 --> 00:10:23.470 I'll abbreviate antigen, AG. 00:10:23.470 --> 00:10:27.220 So they have antigen receptors, meaning 00:10:27.220 --> 00:10:29.680 that they have things on them that 00:10:29.680 --> 00:10:34.510 recognize specific antigens, and antigens are basically 00:10:34.510 --> 00:10:37.510 things that result in an immune response. 00:10:37.510 --> 00:10:39.190 They could be proteins. 00:10:39.190 --> 00:10:54.150 Antigens are substances that activate the immune system. 00:10:59.760 --> 00:11:04.740 That's just immune system, OK? 00:11:04.740 --> 00:11:07.350 Another abbreviation that I'll use 00:11:07.350 --> 00:11:11.220 is when I refer to an antibody, I'm going to abbreviate it, AB. 00:11:20.630 --> 00:11:24.130 All right, so we have these two branches of the immune system, 00:11:24.130 --> 00:11:27.250 and they each have the type of antigen receptor, 00:11:27.250 --> 00:11:30.580 so now I want to go through what these different types 00:11:30.580 --> 00:11:33.750 of antigen receptors look like, OK? 00:11:33.750 --> 00:11:44.770 And I'm going to start with the B-cell antigen receptor, also 00:11:44.770 --> 00:11:53.460 known as the antibody, also known as an immunoglobulin. 00:11:53.460 --> 00:11:58.450 OK, so another-- these are all synonymous, 00:11:58.450 --> 00:12:01.420 but you will see them in different contexts. 00:12:01.420 --> 00:12:04.220 Immunoglobulin is abbreviated IG. 00:12:08.430 --> 00:12:14.170 And what the antibody looks like structurally, 00:12:14.170 --> 00:12:17.250 it looks like this, and I'll just 00:12:17.250 --> 00:12:19.950 draw it out for you down here. 00:12:19.950 --> 00:12:22.080 So I'm drawing a lipid bilayer that 00:12:22.080 --> 00:12:24.430 represents the plasma membrane. 00:12:24.430 --> 00:12:26.730 The outside of the cell is going to be up, 00:12:26.730 --> 00:12:29.140 so that's the exoplasm up here. 00:12:29.140 --> 00:12:31.580 The inside down here is the cytoplasm. 00:12:34.600 --> 00:12:38.230 And this would be a B cell, then, we're talking about here. 00:12:38.230 --> 00:12:43.800 I'm going to draw just a segment of the B cell plasma membrane. 00:12:43.800 --> 00:12:48.540 And the B-- the antibody can have a transmembrane domain 00:12:48.540 --> 00:12:53.220 that spans the plasma membrane, and then there are domains-- 00:12:56.640 --> 00:13:01.050 and what I'm drawing here is a circle, is an IG domain, 00:13:01.050 --> 00:13:03.435 so this is going to equal an IG domain. 00:13:07.460 --> 00:13:12.510 It's just a type of protein fold that is modular, OK? 00:13:12.510 --> 00:13:15.900 So you can see up on my diagram here, right? 00:13:15.900 --> 00:13:20.520 You see these like here there are these two green segments 00:13:20.520 --> 00:13:24.810 labeled V and C. Each of those is a single IG domain. 00:13:24.810 --> 00:13:27.870 OK, it's just a modular fold that 00:13:27.870 --> 00:13:30.720 is separate from the other part of the protein. 00:13:33.400 --> 00:13:40.740 OK, so here we have along-- 00:13:40.740 --> 00:13:43.500 this is one polypeptide chain that 00:13:43.500 --> 00:13:47.650 has a transmembrane domain, and it is inserted into the plasma 00:13:47.650 --> 00:13:48.150 membrane. 00:13:48.150 --> 00:13:52.800 The N-terminus is here, the C-terminus is down here, 00:13:52.800 --> 00:14:01.440 and each antibody protein has two of these long peptides. 00:14:01.440 --> 00:14:04.470 And because they're the longest part of the molecule, 00:14:04.470 --> 00:14:07.410 they're known as heavy chains, so these are the heavy chains. 00:14:10.530 --> 00:14:13.500 And each antibody protein is composed of two 00:14:13.500 --> 00:14:15.840 identical heavy chains, OK? 00:14:15.840 --> 00:14:18.070 So these are identical. 00:14:18.070 --> 00:14:20.550 And then also there's another component, 00:14:20.550 --> 00:14:27.090 which is present up here, and this is a shorter polypeptide. 00:14:27.090 --> 00:14:29.670 And because it's shorter and smaller, 00:14:29.670 --> 00:14:31.460 it's known as the light chain. 00:14:31.460 --> 00:14:32.760 OK, that's the light chain. 00:14:38.320 --> 00:14:42.310 OK, so that's more or less what an antibody looks like. 00:14:42.310 --> 00:14:45.400 The part of this antigen receptor that 00:14:45.400 --> 00:14:49.430 recognizes the antigen are the tips right here, 00:14:49.430 --> 00:14:55.030 so this is where the antigen binds, 00:14:55.030 --> 00:14:58.180 and it can bind on either this side or this side. 00:14:58.180 --> 00:15:01.570 This molecule is laterally symmetric. 00:15:01.570 --> 00:15:03.880 One side is identical to the other, OK? 00:15:06.940 --> 00:15:15.830 Now, the T-cell receptor looks different, 00:15:15.830 --> 00:15:18.790 and the T cell receptor has fewer names. 00:15:18.790 --> 00:15:22.030 It's just called the T-cell receptor, or the TCR, 00:15:22.030 --> 00:15:23.770 for short. 00:15:23.770 --> 00:15:28.450 And the T-cell receptor is structurally very different, 00:15:28.450 --> 00:15:32.860 so now I'm drawing here a T-cell plasma membrane. 00:15:32.860 --> 00:15:34.600 Here's the plasma membrane. 00:15:34.600 --> 00:15:36.970 The exoplasm, again, is up. 00:15:36.970 --> 00:15:42.220 The cytoplasm is down below this plasma membrane. 00:15:42.220 --> 00:15:47.380 And the T-cell receptor has two chains. 00:15:47.380 --> 00:15:51.970 One is called alpha and the other is beta, 00:15:51.970 --> 00:15:55.660 and it has fewer immunoglobulin repeats, 00:15:55.660 --> 00:16:00.730 so that you can see you just have this sort of smaller 00:16:00.730 --> 00:16:04.090 system here, where you have an alpha and a beta chain. 00:16:04.090 --> 00:16:06.760 And in this case, this region here 00:16:06.760 --> 00:16:08.990 recognizes the antigen, OK? 00:16:08.990 --> 00:16:12.280 So basically the T-cell receptor, or the tip of it, 00:16:12.280 --> 00:16:15.880 interacts with the antigen. 00:16:15.880 --> 00:16:20.320 Now, the B-cell receptor, or the antibody, 00:16:20.320 --> 00:16:24.910 has different forms, so let's talk about the different forms. 00:16:27.790 --> 00:16:31.060 And these are shown up on my slide above, right? 00:16:31.060 --> 00:16:34.690 So you see over here, here is an antibody that 00:16:34.690 --> 00:16:37.720 has a transmembrane domain and is anchored in the plasma 00:16:37.720 --> 00:16:40.510 membrane, but there's another form that 00:16:40.510 --> 00:16:43.660 lacks that transmembrane domain, and instead 00:16:43.660 --> 00:16:46.660 of being an integral membrane protein, 00:16:46.660 --> 00:16:51.070 is instead secreted into the blood, OK? 00:16:51.070 --> 00:16:53.770 So the forms of the B cell receptor 00:16:53.770 --> 00:16:59.380 are both a membrane-bound form, which is initially 00:16:59.380 --> 00:17:03.670 how this antibody is presented, but later on, it 00:17:03.670 --> 00:17:06.790 can be secreted, and this often changes 00:17:06.790 --> 00:17:10.480 when there is an infection, OK? 00:17:10.480 --> 00:17:14.810 So once you have a virus or bacteria in your system, 00:17:14.810 --> 00:17:16.750 then you get the B cells sort of pumping out 00:17:16.750 --> 00:17:18.550 the secreted form of the antibody 00:17:18.550 --> 00:17:21.220 in order to fight the infection, OK? 00:17:21.220 --> 00:17:25.780 In contrast, for the T-cell receptor. 00:17:25.780 --> 00:17:28.720 For the T-cell receptor, there's only one form, which 00:17:28.720 --> 00:17:31.210 is the membrane-bound form, OK? 00:17:31.210 --> 00:17:33.910 So for T-cell receptors, it's membrane only. 00:17:39.040 --> 00:17:42.370 OK, another thing that differs between these antigen centers 00:17:42.370 --> 00:17:47.290 receptors is the types of antigens that are recognized. 00:17:47.290 --> 00:17:52.020 So antibodies can recognize all sorts of different molecules, 00:17:52.020 --> 00:17:52.520 OK? 00:17:52.520 --> 00:17:56.290 They're very promiscuous, but they-- 00:17:56.290 --> 00:17:59.180 and a given antibody is not promiscuous. 00:17:59.180 --> 00:18:04.030 A given antibody will recognize a very specific structure, 00:18:04.030 --> 00:18:06.340 but the possibility for antibodies 00:18:06.340 --> 00:18:09.910 is that they can recognize small molecules. 00:18:12.730 --> 00:18:19.330 They can recognize proteins, they can recognize DNA, 00:18:19.330 --> 00:18:21.820 they can recognize carbohydrates, 00:18:21.820 --> 00:18:23.560 you get the idea, right? 00:18:23.560 --> 00:18:28.420 They really can recognize a whole range of different types 00:18:28.420 --> 00:18:29.440 of molecules. 00:18:32.060 --> 00:18:37.550 In contrast, the T-cell receptor is more restricted 00:18:37.550 --> 00:18:42.110 in that T-cell receptors will recognize peptides or short 00:18:42.110 --> 00:18:44.850 sequences of amino acids. 00:18:44.850 --> 00:18:50.240 So it recognizes peptides, and these peptides 00:18:50.240 --> 00:18:59.480 are presented to the T cell on a type of molecule 00:18:59.480 --> 00:19:04.400 presented by the MHC complex. 00:19:04.400 --> 00:19:08.210 There are two classes, 1 and 2, and we're 00:19:08.210 --> 00:19:11.660 going to talk about this in detail in Friday's lecture. 00:19:11.660 --> 00:19:14.330 So I just want to point out the difference 00:19:14.330 --> 00:19:18.080 in the types of antigens that can be recognized here, 00:19:18.080 --> 00:19:22.010 and we'll talk about exactly what that means on Friday. 00:19:26.370 --> 00:19:30.490 OK, so now we have to talk about the amazing properties 00:19:30.490 --> 00:19:31.940 of the immune system. 00:19:31.940 --> 00:19:36.460 The first is how specific it is, its specificity, 00:19:36.460 --> 00:19:41.560 and I think this is a really amazing property, the ability 00:19:41.560 --> 00:19:48.130 to really discriminate between very closely related molecules, 00:19:48.130 --> 00:19:48.940 right? 00:19:48.940 --> 00:19:52.450 And this is essential for immunity to work well. 00:19:52.450 --> 00:19:57.040 You want to recognize things that our foreign agents that 00:19:57.040 --> 00:19:59.500 have like invaded your system. 00:19:59.500 --> 00:20:04.330 You don't want to be recognizing proteins and structures that 00:20:04.330 --> 00:20:06.550 are natively present in your body, 00:20:06.550 --> 00:20:08.890 because if your immune system did that, you'd 00:20:08.890 --> 00:20:12.730 have an autoimmune disease, so this specificity 00:20:12.730 --> 00:20:17.180 is really crucial for the function of the immune system. 00:20:17.180 --> 00:20:20.770 So now I want to talk how is it that the immune system achieves 00:20:20.770 --> 00:20:25.390 such high levels of specificity, and the way 00:20:25.390 --> 00:20:28.990 I want to illustrate this is I want 00:20:28.990 --> 00:20:32.810 to bring this down quickly. 00:20:32.810 --> 00:20:36.470 So if we consider the structure of the antibody, 00:20:36.470 --> 00:20:43.000 these different domains are different in that-- 00:20:43.000 --> 00:20:46.340 in how variable they are, so some are variable. 00:20:46.340 --> 00:20:48.700 So this domain here for the heavy chain 00:20:48.700 --> 00:20:51.520 is the variable domain of the heavy chain, which I'll just 00:20:51.520 --> 00:20:56.260 abbreviate VH, and then these other immunoglobulin domains 00:20:56.260 --> 00:20:59.850 are constant, meaning they don't have a lot of variation 00:20:59.850 --> 00:21:02.320 in sequence. 00:21:02.320 --> 00:21:05.080 Like the heavy chain, there is a variable domain 00:21:05.080 --> 00:21:08.740 for the light chain, which I'll abbreviate VL, 00:21:08.740 --> 00:21:13.840 and then there is a constant domain for the light chain, OK? 00:21:13.840 --> 00:21:17.530 And so what I want to do now is consider 00:21:17.530 --> 00:21:21.790 what the sequence variation is here on this antibody 00:21:21.790 --> 00:21:23.290 is the same over here. 00:21:23.290 --> 00:21:26.450 This is the same thing over here. 00:21:26.450 --> 00:21:28.810 You have a variable domain for the heavy chain 00:21:28.810 --> 00:21:30.910 and a variable domain for the light chain. 00:21:33.680 --> 00:21:39.460 So let's consider the amino acid sequence of the antibody 00:21:39.460 --> 00:21:46.220 molecule specifically at that variable part of the protein. 00:21:46.220 --> 00:21:50.470 So let's say we could take individual antibodies 00:21:50.470 --> 00:21:53.700 and define their sequence from end to C-terminus. 00:21:53.700 --> 00:21:58.160 That would be from tip towards the end here. 00:21:58.160 --> 00:22:01.960 So if we take a number of different antibodies 00:22:01.960 --> 00:22:06.380 and align their amino acid sequence-- 00:22:06.380 --> 00:22:07.210 so what I am-- 00:22:07.210 --> 00:22:09.640 I'm not writing out an amino acid sequence, 00:22:09.640 --> 00:22:12.310 but I'm just illustrating like a particular type 00:22:12.310 --> 00:22:15.310 of computational experiment you could do. 00:22:15.310 --> 00:22:24.880 So these would be aligned amino acid sequences 00:22:24.880 --> 00:22:31.180 where each of these represents a different antibody, let's say, 00:22:31.180 --> 00:22:35.680 heavy chain polypeptide that's produced from a different B 00:22:35.680 --> 00:22:36.700 cell, OK? 00:22:36.700 --> 00:22:45.490 So each of these is a different antibody from a unique B cell. 00:22:51.070 --> 00:22:56.260 And then we just consider the residue number 00:22:56.260 --> 00:22:58.840 and how much each amino acid residue 00:22:58.840 --> 00:23:03.130 varies along this sequence. 00:23:03.130 --> 00:23:07.990 So if we were to align antibody gene stretches like this 00:23:07.990 --> 00:23:10.480 and look at how much variation there is, 00:23:10.480 --> 00:23:13.540 you'd get a graph that looks like this, OK? 00:23:13.540 --> 00:23:17.460 So the y-axis is the amount of variation 00:23:17.460 --> 00:23:21.700 and the x-axis here is the residue number 00:23:21.700 --> 00:23:25.630 along this polypeptide sequence. 00:23:25.630 --> 00:23:30.070 And what you see, probably even without the color here, 00:23:30.070 --> 00:23:32.830 is that there are these three regions where there's 00:23:32.830 --> 00:23:35.110 a lot of variation in the sequence 00:23:35.110 --> 00:23:37.240 of different antibodies, OK? 00:23:37.240 --> 00:23:40.930 So here you see the blue segment here has a lot of variation, 00:23:40.930 --> 00:23:43.330 the yellow segment has a lot of variation, 00:23:43.330 --> 00:23:49.870 and the reds segment here has possibly the most variation. 00:23:49.870 --> 00:23:59.650 And what these are known as are hypervariable regions, 00:23:59.650 --> 00:24:03.080 meaning that they exhibit a lot of variation. 00:24:03.080 --> 00:24:05.470 Another name for them is that they 00:24:05.470 --> 00:24:08.860 are complementarity-determining regions. 00:24:08.860 --> 00:24:10.892 Complementarity. 00:24:10.892 --> 00:24:12.655 Complementarity-determining. 00:24:20.830 --> 00:24:28.930 Determining regions, or CDRs, and there are 00:24:28.930 --> 00:24:33.550 three of them, 1, 2, and 3, OK? 00:24:33.550 --> 00:24:37.090 So there are regions in this antibody molecule 00:24:37.090 --> 00:24:41.140 which are much more variable than others, OK? 00:24:41.140 --> 00:24:43.330 So what are these regions? 00:24:43.330 --> 00:24:48.280 Well, this is a sort of crystal structure of the-- 00:24:48.280 --> 00:24:51.400 of an antibody, and you can see how the antigen is 00:24:51.400 --> 00:24:53.110 bound at the end. 00:24:53.110 --> 00:24:55.330 That would be this end of the molecule 00:24:55.330 --> 00:24:57.520 or this end of the molecule. 00:24:57.520 --> 00:24:59.440 And here you see a ribbon diagram 00:24:59.440 --> 00:25:01.300 of the structure of the antibody, 00:25:01.300 --> 00:25:03.820 and the complementary complementarity 00:25:03.820 --> 00:25:06.790 determining regions are the regions here 00:25:06.790 --> 00:25:10.840 that contact the antigen. 00:25:10.840 --> 00:25:13.090 And what they are are basically here's 00:25:13.090 --> 00:25:17.650 an IG fold, this whole thing, and there are these three loops 00:25:17.650 --> 00:25:20.500 that extend out of the end of this molecule, 00:25:20.500 --> 00:25:23.650 and you can think of them as three fingers, OK? 00:25:23.650 --> 00:25:27.280 Then these fingers are able to reach out and sort of grab on 00:25:27.280 --> 00:25:31.120 to like a foreign particle and/or any particle 00:25:31.120 --> 00:25:34.630 and stick to it, OK? 00:25:34.630 --> 00:25:38.740 So these are the variable regions, 00:25:38.740 --> 00:25:42.040 and they have differences in amino acids-- 00:25:42.040 --> 00:25:45.460 in amino acid sequence, and even very small 00:25:45.460 --> 00:25:56.230 differences in the amino acid sequence 00:25:56.230 --> 00:26:00.010 at this particular part of the antibody 00:26:00.010 --> 00:26:02.770 can have a huge effect on whether or not 00:26:02.770 --> 00:26:04.660 they're able to stick to something, right? 00:26:04.660 --> 00:26:06.670 You can imagine if I lost my thumb, 00:26:06.670 --> 00:26:09.220 then right now, I'm not able to sort of stick 00:26:09.220 --> 00:26:11.290 to that anymore, OK? 00:26:11.290 --> 00:26:14.860 So small differences in amino acid sequence 00:26:14.860 --> 00:26:24.610 result in large changes in the affinity of this antibody 00:26:24.610 --> 00:26:31.630 for an antigen. And antibodies have different sequences, 00:26:31.630 --> 00:26:36.750 meaning that they're able to bind to specific substances 00:26:36.750 --> 00:26:38.490 differently. 00:26:38.490 --> 00:26:40.980 So if an antibody has one set of sequence, 00:26:40.980 --> 00:26:43.260 it might recognize one structure. 00:26:43.260 --> 00:26:45.033 If it has another sequence, it might 00:26:45.033 --> 00:26:46.200 recognize another structure. 00:26:49.170 --> 00:26:52.290 So just by changing the sequence at 00:26:52.290 --> 00:26:54.090 these complementarity-determining 00:26:54.090 --> 00:26:57.630 regions has a huge influence on what these proteins will 00:26:57.630 --> 00:27:01.080 bind to, OK? 00:27:01.080 --> 00:27:05.340 Now, each B cell expresses a unique antibody 00:27:05.340 --> 00:27:08.700 and just one unique antibody. 00:27:08.700 --> 00:27:18.000 So each B cell in our body expresses one and only 00:27:18.000 --> 00:27:23.100 one antibody protein, and that antibody protein 00:27:23.100 --> 00:27:29.630 has a unique sequence at the CDR region, 00:27:29.630 --> 00:27:44.450 and this one antibody has unique specificity for an antigen, OK? 00:27:44.450 --> 00:27:46.970 So here you can see in my diagram, 00:27:46.970 --> 00:27:49.520 I have a whole bunch of B cells here. 00:27:49.520 --> 00:27:53.300 They all express a different antibody, 00:27:53.300 --> 00:27:56.030 and you can see that the way you could get more of a given 00:27:56.030 --> 00:28:01.310 antibody is to clonally expand one of these cells, 00:28:01.310 --> 00:28:05.240 and all of the cells that result from that colonial expansion 00:28:05.240 --> 00:28:07.670 will express the exact same antibody. 00:28:11.330 --> 00:28:15.230 And when you have a clonal population of a cell 00:28:15.230 --> 00:28:23.750 that all has the same antibody, that's known as monoclonal, OK? 00:28:23.750 --> 00:28:28.580 So each B cell will have a B-cell receptor or an antibody 00:28:28.580 --> 00:28:30.930 with unique specificity. 00:28:30.930 --> 00:28:35.120 So now the question becomes, OK, so I told you 00:28:35.120 --> 00:28:37.490 how you get specificity, but in order 00:28:37.490 --> 00:28:39.620 to have a functioning immune system, 00:28:39.620 --> 00:28:42.320 you need to have lots of different cells 00:28:42.320 --> 00:28:47.210 that each express a different cell receptor, 00:28:47.210 --> 00:28:50.480 so there needs to be a way to generate diversity. 00:28:54.800 --> 00:28:57.440 And the answer to how we generate diversity 00:28:57.440 --> 00:29:00.110 has an MIT connection. 00:29:00.110 --> 00:29:04.910 The research wasn't done at MIT, but the person 00:29:04.910 --> 00:29:08.810 who discovered the mechanism is now at MIT. 00:29:08.810 --> 00:29:16.190 This research was performed by Susumu Tonegawa, 00:29:16.190 --> 00:29:19.460 and Professor Tonegawa, for his work 00:29:19.460 --> 00:29:21.980 on how this diversity is generated, 00:29:21.980 --> 00:29:29.640 was awarded the Nobel Prize in medicine in 1987. 00:29:29.640 --> 00:29:33.390 OK, so Professor Tonegawa did this research elsewhere, 00:29:33.390 --> 00:29:36.230 but now he is a faculty member here at MIT. 00:29:41.420 --> 00:29:42.860 All right, so diversity. 00:29:42.860 --> 00:29:44.410 The problem of diversity, right? 00:29:44.410 --> 00:29:48.420 We have millions of B cells that have a unique antibody. 00:29:48.420 --> 00:29:50.950 OK, so one solution to this problem 00:29:50.950 --> 00:29:54.830 would be we have a million different antibody genes, 00:29:54.830 --> 00:29:58.180 and each B cell clone sort of expresses one of them 00:29:58.180 --> 00:30:01.540 OK how many genes do we have? 00:30:01.540 --> 00:30:05.495 Anyone know, roughly, on the order of magnitude? 00:30:05.495 --> 00:30:06.370 Do we have a million? 00:30:06.370 --> 00:30:07.142 What's that? 00:30:07.142 --> 00:30:08.350 AUDIENCE: 30,000? 00:30:08.350 --> 00:30:10.200 ADAM MARTIN: Exactly. 00:30:10.200 --> 00:30:12.850 Yeah, so Mr. George has suggested-- 00:30:12.850 --> 00:30:14.560 Miles, I believe. 00:30:14.560 --> 00:30:15.460 Yeah, OK, good. 00:30:15.460 --> 00:30:20.170 Miles suggested 30,000, which is the good upper limit, right? 00:30:20.170 --> 00:30:23.080 So having a million antibody genes 00:30:23.080 --> 00:30:27.490 sounds a little bit unfeasible, OK? 00:30:27.490 --> 00:30:30.490 And so it's basically unfeasible for us 00:30:30.490 --> 00:30:33.760 to express as many antibody genes 00:30:33.760 --> 00:30:37.330 or have as many antibody genes as we have antibodies. 00:30:37.330 --> 00:30:41.350 We just don't have enough real estate in our genome, OK? 00:30:41.350 --> 00:30:43.840 But there's another solution to generate 00:30:43.840 --> 00:30:46.510 the diversity, which is essentially 00:30:46.510 --> 00:30:48.760 a form of shuffling. 00:30:48.760 --> 00:30:55.430 So we have a single heavy chain gene for antibodies, 00:30:55.430 --> 00:30:58.060 and we have two genes for the light chain, 00:30:58.060 --> 00:31:04.090 but these genes are composed of multiple gene segments. 00:31:04.090 --> 00:31:06.340 There are multiple gene segments. 00:31:09.580 --> 00:31:12.250 Specifically, the segments that make up-- 00:31:15.120 --> 00:31:18.990 that generate this variable domain is composed 00:31:18.990 --> 00:31:23.310 of multiple gene segments, and these gene-shaped segments 00:31:23.310 --> 00:31:27.030 are shuffled during the development of the B cell 00:31:27.030 --> 00:31:29.700 to give rise to different proteins. 00:31:29.700 --> 00:31:36.390 OK, so these gene segments are shuffled to generate 00:31:36.390 --> 00:31:37.380 this diversity. 00:31:43.790 --> 00:31:47.550 OK, so now I'm showing you on the top here, 00:31:47.550 --> 00:31:53.300 this is the human immunoglobulin heavy chain locus here. 00:31:53.300 --> 00:31:54.600 You can see it's pretty big. 00:31:54.600 --> 00:31:56.310 There are lots of components. 00:31:56.310 --> 00:31:59.070 I want you to focus on this. 00:31:59.070 --> 00:32:04.020 So there is-- you see in orange, there's this variable gene 00:32:04.020 --> 00:32:08.820 segment, and there are 45 variable gene segments here. 00:32:08.820 --> 00:32:11.190 There's this diversity, or D segment 00:32:11.190 --> 00:32:15.360 here, which there are 23 of, and then there 00:32:15.360 --> 00:32:20.480 are six of these joining or J segments. 00:32:20.480 --> 00:32:25.020 OK, so these are all distinct parts of the gene. 00:32:25.020 --> 00:32:27.600 They're all distinct parts of the exon 00:32:27.600 --> 00:32:31.360 that encodes this variable region of the antibody, OK? 00:32:34.680 --> 00:32:44.835 So you have multiple V, D, and J gene segments. 00:32:49.410 --> 00:32:53.220 And in order to generate a functional antibody, 00:32:53.220 --> 00:32:56.820 one V has to be brought together with one D, which 00:32:56.820 --> 00:33:02.970 has to be brought together with one J for that heavy chain, OK? 00:33:02.970 --> 00:33:06.600 So you have multiple V-D gene segments, 00:33:06.600 --> 00:33:14.700 and they have to be brought together 00:33:14.700 --> 00:33:18.767 to form a functional antibody. 00:33:22.510 --> 00:33:25.540 OK, that's illustrated right here. 00:33:25.540 --> 00:33:27.390 So here you see this is the light chain. 00:33:27.390 --> 00:33:30.630 For the light chain, there are only V and J gene segments. 00:33:30.630 --> 00:33:35.310 V For the heavy chain, there there's V, D, and J. 00:33:35.310 --> 00:33:39.040 And so most of the cells in our body 00:33:39.040 --> 00:33:42.390 and the cells of our germline, at the very earliest 00:33:42.390 --> 00:33:45.460 stages of development, all have this arrangement, 00:33:45.460 --> 00:33:48.270 where you have everything still intact. 00:33:48.270 --> 00:33:50.130 But during lymphocytes development, 00:33:50.130 --> 00:33:54.960 specifically in lymphocytes, there is a recombination event 00:33:54.960 --> 00:33:58.675 that brings together V and J segments or V, D, 00:33:58.675 --> 00:34:01.740 and J segments, OK? 00:34:01.740 --> 00:34:09.420 So this is mediated by recombination 00:34:09.420 --> 00:34:23.219 at the heavy and light chain genes for that antibody, OK? 00:34:23.219 --> 00:34:26.010 And so this is very different from the recombination we 00:34:26.010 --> 00:34:29.520 talked about earlier in the semester, where recombination 00:34:29.520 --> 00:34:33.159 is happening during meiosis and the formation of the gametes, 00:34:33.159 --> 00:34:33.659 right? 00:34:33.659 --> 00:34:36.239 In that case, recombination is happening 00:34:36.239 --> 00:34:38.550 between homologous chromosomes. 00:34:38.550 --> 00:34:41.280 Here we're not talking about recombination between 00:34:41.280 --> 00:34:43.330 homologous chromosomes. 00:34:43.330 --> 00:34:45.239 We're talking about recombination 00:34:45.239 --> 00:34:49.170 that brings together and deletes segments 00:34:49.170 --> 00:34:52.650 along a single chromosome to bring these V and J 00:34:52.650 --> 00:34:54.600 segments together, OK? 00:34:54.600 --> 00:34:56.820 So this is sort of a intra-chromosomal 00:34:56.820 --> 00:35:01.350 recombination, which deletes the intervening sequences 00:35:01.350 --> 00:35:05.280 and brings these gene segments together to form a functional 00:35:05.280 --> 00:35:07.540 antibody protein. 00:35:07.540 --> 00:35:17.040 So this process is known as V(D)J recombination, 00:35:17.040 --> 00:35:18.765 and this is lymphocyte specific. 00:35:25.310 --> 00:35:29.710 OK, and that's because during the development of B and T 00:35:29.710 --> 00:35:36.370 cells, there is an induction of recombinases that 00:35:36.370 --> 00:35:39.260 mediate this recombination. 00:35:39.260 --> 00:35:45.830 So in this case, there is recombination, 00:35:45.830 --> 00:35:50.890 which is mediated by recombination-activating genes 00:35:50.890 --> 00:35:55.660 1 and 2, called RAG1 and 2, OK? 00:35:55.660 --> 00:35:58.030 So there are these are lymphocyte-specific 00:35:58.030 --> 00:36:02.050 recombinases which mediate this rearrangement, which 00:36:02.050 --> 00:36:07.270 bring together a unique V, D, and J segments together, OK? 00:36:07.270 --> 00:36:09.820 So the diversity comes from the fact 00:36:09.820 --> 00:36:21.625 that each of these V, D and J segments, each V segment-- 00:36:24.460 --> 00:36:27.730 you could-- this also applies to D segments and also J 00:36:27.730 --> 00:36:28.750 segments-- 00:36:28.750 --> 00:36:29.710 has a unique sequence. 00:36:35.050 --> 00:36:38.740 So it encodes for a unique amino acid sequence, 00:36:38.740 --> 00:36:40.720 meaning that if you bring together 00:36:40.720 --> 00:36:44.260 different combinations of Vs, Ds, and Js, 00:36:44.260 --> 00:36:47.110 you get a distinct protein, OK? 00:36:47.110 --> 00:36:51.760 Now even if you had all of the combinations of V, Ds, and Js, 00:36:51.760 --> 00:36:53.650 you still don't have the diversity 00:36:53.650 --> 00:36:55.570 that we see in the human body. 00:36:55.570 --> 00:36:59.900 So there is another process that further generates diversity, 00:36:59.900 --> 00:37:02.260 which is the fact that when these segments are getting 00:37:02.260 --> 00:37:07.570 shuffled, it's imprecise in that nucleotides can be inserted 00:37:07.570 --> 00:37:11.000 or deleted as these segments are joined, 00:37:11.000 --> 00:37:14.402 which generates greater amino acid diversity, 00:37:14.402 --> 00:37:15.235 and this is called-- 00:37:19.720 --> 00:37:22.123 it's called junctional imprecision. 00:37:26.860 --> 00:37:29.960 So this recombination is not precise, 00:37:29.960 --> 00:37:35.780 but it leads to the insertion or deletion 00:37:35.780 --> 00:37:41.420 of nucleotides of nucleotides. 00:37:47.150 --> 00:37:52.220 And if there's a multiple of 3 nucleotides 00:37:52.220 --> 00:37:57.380 either inserted or deleted, then you get a functional antibody. 00:37:57.380 --> 00:38:01.260 Why is it that it has to be a multiple of 3? 00:38:01.260 --> 00:38:01.760 Jeremy? 00:38:01.760 --> 00:38:03.880 AUDIENCE: Otherwise, you end up with a frameshift mutation. 00:38:03.880 --> 00:38:04.480 ADAM MARTIN: Exactly. 00:38:04.480 --> 00:38:04.980 Right? 00:38:04.980 --> 00:38:06.250 This is and the-- 00:38:06.250 --> 00:38:09.430 this is on the more sort of like on the N-terminus 00:38:09.430 --> 00:38:11.170 side of the gene, right? 00:38:11.170 --> 00:38:16.030 So if you inserted one nucleotide between V and J, 00:38:16.030 --> 00:38:19.420 then the downstream portion of the gene, the downstream part 00:38:19.420 --> 00:38:22.260 of the open reading frame would be out of frame 00:38:22.260 --> 00:38:24.630 and wouldn't generate a functional protein. 00:38:24.630 --> 00:38:27.340 OK, so it has to be a multiple of 3. 00:38:30.140 --> 00:38:30.800 Yeah, Georgia? 00:38:30.800 --> 00:38:34.010 AUDIENCE: How is functional precision lymphocyte-specific? 00:38:34.010 --> 00:38:35.260 Or is it not? 00:38:35.260 --> 00:38:37.550 ADAM MARTIN: It's just the RAG1 and RAG2 00:38:37.550 --> 00:38:42.239 are turned on specifically in the lymphocytes as they mature. 00:38:42.239 --> 00:38:45.140 AUDIENCE: And that also affects the insertion, deletion? 00:38:45.140 --> 00:38:47.265 ADAM MARTIN: Well, if you don't have recombination, 00:38:47.265 --> 00:38:49.580 you can't get junctional precision, right? 00:38:49.580 --> 00:38:52.580 So the junctional imprecision-- or junctional imprecision. 00:38:52.580 --> 00:38:55.610 The junctional imprecision is a consequence 00:38:55.610 --> 00:38:59.000 of the recombination process itself, right? 00:38:59.000 --> 00:39:01.100 So if you're not having recombination, 00:39:01.100 --> 00:39:03.350 you're not having any junctional imprecision 00:39:03.350 --> 00:39:06.750 because you're not generating a junction. 00:39:06.750 --> 00:39:10.130 OK, now there's one more thing that's important here, 00:39:10.130 --> 00:39:13.820 which is something that happens not 00:39:13.820 --> 00:39:17.220 as a consequence of this recombination process 00:39:17.220 --> 00:39:20.390 but as a consequence of activating the T cell 00:39:20.390 --> 00:39:24.950 response, which is that in addition to these variations, 00:39:24.950 --> 00:39:29.240 there's also something known as somatic mutation. 00:39:34.340 --> 00:39:40.400 So there's an elevated mutation rate at the IG locus 00:39:40.400 --> 00:39:44.270 that further increases the diversity of the amino acid 00:39:44.270 --> 00:39:49.250 sequence at these variable regions of the antibody, OK? 00:39:49.250 --> 00:39:52.130 Another way this is referred to is 00:39:52.130 --> 00:39:55.580 because it can increase the affinity of the antibody 00:39:55.580 --> 00:40:02.090 for a antigen, it's also known as affinity maturation, 00:40:02.090 --> 00:40:05.540 so these are synonymous. 00:40:05.540 --> 00:40:08.120 Maturation. 00:40:08.120 --> 00:40:10.770 Maturation. 00:40:10.770 --> 00:40:14.390 OK, so-- and this depends on the T 00:40:14.390 --> 00:40:18.870 cell, the cell-mediated branch of adapted immunity, 00:40:18.870 --> 00:40:20.105 so this is T-cell mediated. 00:40:30.890 --> 00:40:34.370 So one other aspect of this process 00:40:34.370 --> 00:40:39.320 that I want to talk about is until this recombination 00:40:39.320 --> 00:40:43.700 happens, the immunoglobulin gene is not expressed, 00:40:43.700 --> 00:40:46.820 so it's this recombination that leads 00:40:46.820 --> 00:40:50.960 to the expression of the-- either the heavy chain 00:40:50.960 --> 00:40:53.660 or the light chain gene, OK? 00:40:53.660 --> 00:40:57.770 And that's because the enhancer is sort of downstream 00:40:57.770 --> 00:41:01.440 in the gene, and by deleting the intervening sequence here, 00:41:01.440 --> 00:41:04.220 you bring the promoter in range of the enhancer, 00:41:04.220 --> 00:41:07.880 and now this gene is expressed, OK? 00:41:07.880 --> 00:41:10.980 But remember you have two copies of each of these genes. 00:41:10.980 --> 00:41:13.640 You have a parental copy and a maternal copy, 00:41:13.640 --> 00:41:16.190 and another feature of this system 00:41:16.190 --> 00:41:19.070 is that there is what is known as allelic exclusion. 00:41:24.380 --> 00:41:29.900 So the system is such that a B cell expresses only one 00:41:29.900 --> 00:41:33.830 antibody, and so if you had both alleles expressing, 00:41:33.830 --> 00:41:35.900 that wouldn't be the case, OK? 00:41:35.900 --> 00:41:40.700 So allelic exclusion makes it that if you get a recombination 00:41:40.700 --> 00:41:44.960 event that leads to a functional antibody for one 00:41:44.960 --> 00:41:49.850 of your sort of inherited copies of the gene, one 00:41:49.850 --> 00:41:53.850 of your alleles, it suppresses recombination on the other one, 00:41:53.850 --> 00:41:54.350 OK? 00:41:54.350 --> 00:41:58.460 So you will only get one of these genes, one heavy chain 00:41:58.460 --> 00:42:01.370 and one light chain, expressed per B cell. 00:42:01.370 --> 00:42:12.260 OK, so only one gene expressed so that each B cell only 00:42:12.260 --> 00:42:13.850 has one antibody. 00:42:19.690 --> 00:42:23.290 OK, I just wanted to point out, finally, 00:42:23.290 --> 00:42:28.270 that these junctions between V-D and J segments 00:42:28.270 --> 00:42:31.600 fall right in this CDR-3 region, so they're 00:42:31.600 --> 00:42:34.870 responsible for the high level of variability 00:42:34.870 --> 00:42:40.840 at the CDR or hypervariable 3 region. 00:42:40.840 --> 00:42:43.690 OK, and because of the allelic exclusion, 00:42:43.690 --> 00:42:48.700 each B cell expresses only one antibody, OK? 00:42:48.700 --> 00:42:51.730 So all of the antibody proteins expressed by that cell 00:42:51.730 --> 00:42:55.060 will be exactly the same. 00:42:55.060 --> 00:42:59.620 OK, so now the last property of the immune system we 00:42:59.620 --> 00:43:00.970 need to talk about is memory. 00:43:06.970 --> 00:43:10.120 And so the immune system needs to be 00:43:10.120 --> 00:43:16.450 able to recall past infectious agents that it's experienced, 00:43:16.450 --> 00:43:18.010 and so it needs-- 00:43:18.010 --> 00:43:20.050 I guess we're kind of personifying here, 00:43:20.050 --> 00:43:22.090 but it needs some sort of memory, right? 00:43:22.090 --> 00:43:26.795 It needs the ability to recall this, OK? 00:43:26.795 --> 00:43:29.620 And this is the principle behind vaccination, right? 00:43:40.190 --> 00:43:43.340 The way vaccines work is to put in one 00:43:43.340 --> 00:43:47.270 of these attenuated or inactivated foreign agents, 00:43:47.270 --> 00:43:49.610 such that your body is able to remember that 00:43:49.610 --> 00:43:51.710 later on when you get the real deal, 00:43:51.710 --> 00:43:55.040 and it's able to fight it off, OK? 00:43:55.040 --> 00:43:57.800 So the body has to be able to remember. 00:43:57.800 --> 00:44:00.920 And several ways in which this manifests itself, 00:44:00.920 --> 00:44:04.280 if we compare a primary infection, the first time 00:44:04.280 --> 00:44:08.870 you've seen an infectious agent, versus a secondary infection, 00:44:08.870 --> 00:44:11.600 they have very different responses 00:44:11.600 --> 00:44:17.600 from the standpoint of the adaptive immune system, OK? 00:44:17.600 --> 00:44:21.500 So if we consider the lag before your adaptive immune system 00:44:21.500 --> 00:44:25.970 really takes off, the primary response 00:44:25.970 --> 00:44:30.410 takes about five to 10 days, so it's a bit delayed, 00:44:30.410 --> 00:44:34.490 whereas the secondary response can be one to three days, OK? 00:44:34.490 --> 00:44:35.700 So it's faster. 00:44:35.700 --> 00:44:39.080 It's able to react faster when you see an infectious agent 00:44:39.080 --> 00:44:41.090 the second time. 00:44:41.090 --> 00:44:45.200 If we also just consider the magnitude of the response 00:44:45.200 --> 00:44:47.870 by considering how much antibody, the antibody 00:44:47.870 --> 00:44:52.230 concentration that's like put into your system, 00:44:52.230 --> 00:44:56.090 then the primary response is smaller 00:44:56.090 --> 00:44:59.150 and the magnitude of the secondary response is larger. 00:45:02.180 --> 00:45:05.690 So you basically-- your body's able to produce more antibody 00:45:05.690 --> 00:45:09.920 against an infectious agent the second time it sees it. 00:45:09.920 --> 00:45:14.480 Not only is the antibody amount better the second time, 00:45:14.480 --> 00:45:17.090 but actually the antibodies themselves 00:45:17.090 --> 00:45:19.610 are better antibodies, OK? 00:45:19.610 --> 00:45:26.750 And we can show that by thinking about antibody affinity, which 00:45:26.750 --> 00:45:30.560 is how tightly the antibody recognizes the antigen, 00:45:30.560 --> 00:45:32.660 and I'll give you numbers that represent 00:45:32.660 --> 00:45:38.970 the dissociation constant for an antibody to a given antigen. 00:45:38.970 --> 00:45:42.770 So the lower that number is, the tighter the binding. 00:45:42.770 --> 00:45:47.030 So for the primary infection, the antibody affinity 00:45:47.030 --> 00:45:52.520 is weaker on the order of 10 to the negative 7th molar in terms 00:45:52.520 --> 00:45:55.760 of KD, and this secondary infection 00:45:55.760 --> 00:45:58.340 generates antibodies that are functionally quite better. 00:45:58.340 --> 00:45:59.405 They bind much tighter. 00:46:02.210 --> 00:46:06.350 It can be less than 10 to the negative 11th molar, which 00:46:06.350 --> 00:46:07.530 is sub-nanomolar. 00:46:07.530 --> 00:46:08.030 Right? 00:46:08.030 --> 00:46:10.160 That's a really tight interaction 00:46:10.160 --> 00:46:12.270 between two molecules. 00:46:12.270 --> 00:46:14.300 So the antibodies, you get more of them, 00:46:14.300 --> 00:46:16.670 and they're better antibodies, OK? 00:46:19.470 --> 00:46:25.860 So what makes this memorable is that when-- what lasts 00:46:25.860 --> 00:46:29.700 in your body from the first time you see the agent to the next 00:46:29.700 --> 00:46:40.700 is there's a type of B cell known as a memory B cell, 00:46:40.700 --> 00:46:46.030 and this memory B cell will express a given antibody, 00:46:46.030 --> 00:46:49.760 and that antibody will be specific to the substance 00:46:49.760 --> 00:46:51.480 you saw previously. 00:46:51.480 --> 00:46:54.350 And because recombination is-- this recombination 00:46:54.350 --> 00:46:57.390 is irreversible, then that B cell 00:46:57.390 --> 00:47:00.140 is going to remember that antibody because it's still 00:47:00.140 --> 00:47:02.240 encoded in the genome. 00:47:02.240 --> 00:47:08.480 So the memory results from V(D)J recombination being 00:47:08.480 --> 00:47:16.130 irreversible and the fact that these memory B cells stay 00:47:16.130 --> 00:47:19.830 in your body, even if the antigen is not present, 00:47:19.830 --> 00:47:24.290 so these also stay in the body. 00:47:32.010 --> 00:47:35.360 OK, so effective vaccines generate these types 00:47:35.360 --> 00:47:37.760 of cells, these memory B cells. 00:47:37.760 --> 00:47:40.990 OK, that's important if you want an effective vaccine, 00:47:40.990 --> 00:47:43.940 that you have these B cells that retain information 00:47:43.940 --> 00:47:47.420 about the past infection. 00:47:47.420 --> 00:47:51.980 All right, so what exactly is it that the antibodies do? 00:47:51.980 --> 00:47:58.970 So I'll talk about effector functions of antibodies. 00:48:01.920 --> 00:48:06.740 So antibodies can bind to a foreign substance 00:48:06.740 --> 00:48:09.830 and interfere with the normal function, right? 00:48:09.830 --> 00:48:12.560 If you have a bacteria and maybe the antibody 00:48:12.560 --> 00:48:15.320 binds to some part of the bacteria 00:48:15.320 --> 00:48:18.710 to interfere with that bacteria getting into the cell, 00:48:18.710 --> 00:48:22.250 and this type of effect is known as neutralization. 00:48:26.870 --> 00:48:29.060 If you had an antibody that bound to something 00:48:29.060 --> 00:48:31.700 like a bacteria, you could also have 00:48:31.700 --> 00:48:37.350 it recruit phagocytic cells to internalize that bacteria, 00:48:37.350 --> 00:48:43.190 and so you could also induce phagocytosis. 00:48:43.190 --> 00:48:47.090 In addition, antibodies, when bound to a foreign substance, 00:48:47.090 --> 00:48:49.730 if that foreign substance is a cell, 00:48:49.730 --> 00:48:54.650 then it could recruit a killing cell to kill that cell, 00:48:54.650 --> 00:48:59.750 so there's also a killing aspect to this, OK? 00:48:59.750 --> 00:49:03.590 So what's in this diagram here is a type of cell 00:49:03.590 --> 00:49:06.920 known as a natural killer cell that is killing its target 00:49:06.920 --> 00:49:09.020 cell, and so you can kind of think 00:49:09.020 --> 00:49:12.320 of this cell as the Terminator, OK? 00:49:12.320 --> 00:49:16.280 So right, if the natural killer cell recognizes this target 00:49:16.280 --> 00:49:19.910 here, then it's hasta la vista, baby, 00:49:19.910 --> 00:49:23.690 and that cell is dead, OK? 00:49:23.690 --> 00:49:27.780 I just want to point out one thing that I mentioned before, 00:49:27.780 --> 00:49:32.300 which is that antibodies can be leveraged to generate 00:49:32.300 --> 00:49:35.970 treatments for certain types of diseases. 00:49:35.970 --> 00:49:39.260 And we talked about a drug called Herceptin-- or not 00:49:39.260 --> 00:49:41.570 a drug but a-- it's an antibody, but it's 00:49:41.570 --> 00:49:47.310 a treatment for HER2 positive breast cancer, 00:49:47.310 --> 00:49:52.670 so this is used to treat HER2-positive breast cancer. 00:49:52.670 --> 00:49:58.580 And it's really been a nice success story in the cancer 00:49:58.580 --> 00:50:02.300 field because what this-- 00:50:02.300 --> 00:50:06.170 what Herceptin is-- it was derived from a mouse antibody, 00:50:06.170 --> 00:50:08.840 so this is a mouse monoclonal antibody 00:50:08.840 --> 00:50:12.050 that recognizes this HER2 growth factor 00:50:12.050 --> 00:50:14.060 receptor, which is over expressed 00:50:14.060 --> 00:50:17.970 on 30% of human breast cancers. 00:50:17.970 --> 00:50:22.430 And what Herceptin is is that researchers took this mouse 00:50:22.430 --> 00:50:26.660 antibody and engineered a human antibody 00:50:26.660 --> 00:50:30.800 to have the mouse sequence at its complementarity-determining 00:50:30.800 --> 00:50:34.370 regions, such that you have a human antibody that 00:50:34.370 --> 00:50:38.990 won't be sort of removed by the human immune system 00:50:38.990 --> 00:50:44.150 but will recognize HER2 and recruit human immune cells 00:50:44.150 --> 00:50:48.440 to HER2 positive cells, possibly killing those cells 00:50:48.440 --> 00:50:52.700 or binding to HER2 and somehow neutralizing the activity 00:50:52.700 --> 00:50:55.760 of HER2 on these cancer cells. 00:50:55.760 --> 00:50:59.570 So antibodies can be very useful for therapeutics, as well 00:50:59.570 --> 00:51:03.990 as being useful in our own bodies to mediate immunity. 00:51:03.990 --> 00:51:06.890 OK, we'll talk about T cells on Friday. 00:51:06.890 --> 00:51:09.400 Remember to bring your projects.