Subtitles section Play video Print subtitles >>Dr. Ketchum: This is the part three of six for Chapter 23: The Immune System. And in this chapter, we’re going to look at characteristics of the B and T cells in this particular part, in part three. So remember that there are two different types of immunity: there’s innate immunity and there’s acquired immunity. So here we’re focusing now on acquired or adapted immunity. Remember that these are slower responses than an innate response and the responses to very specific microbes. So before we actually get into the different types of responses, what we need to do first is to kind of look at the outline of where we’re going to go in this particular section. So we’re dealing with the adaptive immune response, and this actually involves two different types of immunity. So you have what’s called humoral immunity and cell-mediated immunity. If it’s humoral immunity, we will always be working with B lymphocytes. Cell-mediated immunity, on the other hand, you will always be dealing with T lymphocytes. So these two different types of immunity involve lymphocytes, howev—however they involve different types of lymphocytes. So what we’re going to be doing first is looking at characteristics for these lymphocytes before we get into the process of how they actually function as part of the immune system. So first off, let’s look at the origin for the B and the T cells. And so in Chapter 15, which was on the blood, we talked about how various blood cells are produced. They’re a precursors stem cell or a hematopoietic precursor cell, and we talked about how those can give rise to red blood cells, platelets, monocytes, granulocytes, and so on. But now what our focus is going to be on is how those hematopoietic stem cells or precursor cells become lymphoid stem cells, and then those lymphoid stem cells become lymphocytes. And that occurs in the bone marrow. So once we have these lymphocytes then they can start differentiating, and this is where they can differentiate into B and to T lymphocytes. So first let’s focus on the B lymphocytes. The B lymphocytes—they’re pretty straightforward in the sense that they’re going to differentiate and mature in the bone marrow. One they mature into these B cells, then the B cells are going to move into the peripheral lymphoid tissues. And when they move into the peripheral lymphoid tissues, we’re talking about them moving into the spleen, Peyer’s patches, lymph nodes, and tonsils. Just to give you an example of some of your peripheral lymphoid tissues. Once the B cells invade these lymphoid tissues, they’re going to hang out. They’re going to stay in that area, and they’re just going to veg there, okay? So once they’re there, they’re waiting. And what they’re waiting for is an invasion by some foreign component. So once there’s an invasion—let’s say it could be a bacterial invasion, for example—the B cells then leave the peripheral lymphoid tissues. Not all of them at once are going to leave. You’ll always have some left behind, but once those—some of those B cells have left, what they’re going to do then is they’re going to mount what’s called an antibody-mediated immune response. So right away you need to link B cells and the fact that they’re going to mount or cause, in other words, an antibody-mediated immune response. Now the T cells work a little differently. So what T cells are going to do, T cells— you actually have the immature T cells that are going to leave the bone marrow. And when the immature T cells leave the bone marrow, they’re going to go to the thymus. And if you recall, we briefly mentioned the thymus when we talked about hormones. And we talked about one hormone in particular that suppresses the thymus. And if you recall, that hormone was what? The hormone that suppresses the thymus is cortisol. Okay, so what’s going on in the thymus then? When these immature T cells arrive in the thymus, they get trained and tested before they’re able to leave. So what I mean by training and testing them—I’m going to come off over here to the left a little bit—and here’s the thymus. So what we’re going to do is we’re going to take some T cells. So we’ve got a T lymphocyte, which is the same thing as just saying a T cell; T lymphocyte and T cell are the same thing. We’re going to present this T cell on this side with what’s called a non-self cell. In other words, it’s a cell that’s not part of the body. So the T cell should recognize that non-self cell as foreign. And if the T cell recognizes that non-self cell as foreign, then it should kill the non-self cell. It should attack it. If it attacks and kills that non-self cell, then the T cell graduates. It’s passed its test, all right? So if it graduates then it gets to move on to the peripheral lymphoid tissues, all right? And if it gets to go into the peripheral lymphoid tissues it’s going to do what the B cells do. It’s going to hang out there, and it’s going to wait for a foreign invasion. Once that occurs, T cells will move out of the lymphoid tis—tissues, and they’re going to mount what’s called a cell-mediated immune response. We’ll talk in more in more detail about the cell-mediated response as well as the antibody-mediated responses later. Now let’s go back to the thymus again, because the T cell, like you said earlier, is presented with a non self-cell. The T cell can also be presented with a self-cell. So what I mean by a self-cell is this has to be a cell that is normally found within the body. So should the T cell kill it? The answer is no; you do not want a T cell killing self-cells, because then you would have an autoimmune disease. So when the T cell is presented the self-cell, it should not kill the self-cell. And if it does not kill the self-cell— guess what? It gets to graduate and it moves on to the peripheral lymphoid tissues. Okay, but here’s the issue. Let’s say when the T cell is presented with a self-cell it kills the self-cell. Is that a good thing or a bad thing? Okay that’s a bad thing, right, because now you would have an autoimmune disease. So should the T cell graduate? Should it be able to leave the thymus and enter into the peripheral lymphoid tissues? Would that be a good idea? The answer is no. You don’t want a T cell now circulating throughout your body that’s killing self-cells; you would be in real trouble if that would happen. So what should happen now to that T cell? The T cell is going to commit suicide. So the T cell undergoes apoptosis. So this is programmed cell death, and you should’ve learned about programmed cell death in your Introductory Biology course. Now if you haven’t that’s not a problem, just remember that apoptosis is programmed cell death. So that T cell dies. It commits suicide. And it should, because if it doesn’t you will have an autoimmune disease. Okay, so what we have then is the thymus training and testing the T cells. Thymus trains and tests T cells. If the T cells pass the test, so they graduate, then they get to move on to the peripheral lymphoid tissues, peripheral lymphoid tissues, and then they get to leave those whenever there’s a foreign invasion, and then they will mount a cell-mediated immune response. All right, so those are the origins for your B and your T cells. Now what this table does here for you guys is it’s going to compare your B and T lymphocytes. And in the left-hand column, you have characteristics for each of the B and the T lymphocytes. So we’re going to actually be going through each one of these, so you may want to expand on this table and use it as a study guide. I think it’d be a nice comparison for you. All right, so before we go any further, we need to talk about antibodies and antigens. So the first question is, well what is an antigen? An antigen is a foreign molecule or an abnormal cell. So let’s expand on that before we go any further. So if we say an antigen is a foreign molecule, that could be a protein, could be a polysaccharide. And these are protein or polysaccharide components of a foreign cell. Okay, so these are foreign molecules. Proteins, polysaccharides, that’re components of some sort of foreign cell. All right, now what we mean by “of some foreign cell,” you can think about the, a foreign cell as being a virus. You could think about it as being a bacteria, you could think about this as being a—let’s see—fungus, a protist, a parasitic worm. Those are just to give you examples of foreign cells and what we’re talking about here. Now abnormal cells, on the other hand, we may be referring to a tumor cell, because a tumor cell would be considered an abnormal cell, or it could even be a transplanted cell. So if you’ve had an organ transplant, it comes with cells, right? So transplanted cells would be considered abnormal because they’re not normally found in your body. Now regardless of whether or not your antigen is this foreign molecule or it’s an abnormal cell, they all contain what’s called an epitope. So epitopes are recognition sites, which are called antigenic determinants. So if you look at the diagram on the right, here’s the antigen—that’s the cell. The epitopes here are on the cell surface. So the epitope is what binds to the antibody. So you can see the epitope here, and then you can see it binding to the antibody. And when it binds to the antibody, that’s going to trigger an immune response. So let’s take a look at the properties of an antibody. So antibodies are all Y shaped, and you can see the image here on the right-hand side of your screen for the Y-shaped antibody. And you have four interlinked polypeptide chains. So when you look at these, there’s two heavy and long chains, there’s two short and light chains. They have variable and constant regions. What’s really important is that when you look at the antigen-binding site is located, that’s on the end with the variable region. Hopefully that would make sense to you, because the antigen-binding sites combine to a variety of antigens depending upon what antibody you’re working with. So antibodies are specific; they have specific antigen-binding sites. And the reason they can have that is because that site has a variable polypeptide region. All right, so the antigen-biding site will bind to the antigenic determinate. In other words, it will combine into the epitope. And when it binds again, that’s what’s going to trigger an immune response. Now if you remember from lab we’ve talked about blood antigens and blood antibodies. This is a similar concept here, and—except that you’re born with or you’ve inherited your blood antigens, and you inherited your blood antibodies. The difference in this chapter is we’re going to be referring to antigens as something that is foreign. You’re not born with these things, okay? And then the, the antibodies you’re not born with either, because we’re not dealing with blood antibodies or blood antigens here. This is completely different. We’re talking about foreign things that your body is exposed to, and in response, your body will, will make antibodies to that. Okay, so in terms of some more characteristics or additional characteristics of B and T cells, they do illustrate specificity, okay? So B and T cells can both bind to an antigen. But the way that they do this is somewhat different. So let’s look at the way that B and T cells bind to antigens and what they use to bind to antigens. Now we’ve talked a lot about receptors this semester, and so we’re going to return back to the concept of receptors. If you have a B cell, as shown on the left, your B cell antigen receptor is going to be a membrane antibody, okay? Or what’s called a membrane immunoglobulin. This is what binds the antigen, and here is that membrane antibody—right there. So with a B cell, a B cell is capable of binding or the B cell receptor is capable of binding two different—or two, not two different—but two antigens. Then we look at the T cell. So the T cell on the right also has an antigen receptor, but this time, this antigen receptor is a little bit different. It’s not a membrane antibody. So it’s not Y shaped. Okay, this time we’re going to call this a T cell receptor. TCR—T cell receptor. So by looking at that receptor, how many antigens can this thing bind at once? The answer is one; it can only bind one antigen at one time, whereas the B cell could bind two antigens at one time. Okay, so their, their receptors are a little bit different when you talk about B cells versus T cells. Now in terms of diversity, you can have one B cell or T cell and it can have a hundred thousand antigen receptors. So what that means then is B cells and T cells can bind a lot of different antigens. Next then what we have is the concepts of memory. Both B cells and T cells have a memory, so let’s look at primary versus secondary responses. And so what we see here—we want to focus first on the primary response. Look at your axes, okay? Your x-axis is time after infection, and that’s in days. Your antibody concentration is on the y-axis. So let’s say at day is zero you get exposed to some antigen; so you’re exposed to some foreign molecule. All right, so how long is it going to take for your body to produce antibodies? Now this graph shows 10 days to make antibodies, where technically anywhere between seven to 10 days it will take your body to make antibodies, seven to 10 days to make antibodies.