21. The Immune System
T Cells
1
concept
T Cells
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Hi in this video I'm gonna be talking about T cells. So T cells are a main immune cell. Um part of the adaptive immune system that are used to target and kill pathogens. So first they have to be activated they're not normally activated. So the type of cell that activates a T. Cell is called an antigen presenting cell. So what this does is it exactly what it sounds like it takes a pathogen. It chops it upgrades it a little and takes those little antigens present them to T cells and that results in activation. So a good example of this are dendritic cells which are part of the innate immune system. But other cells can do this as well. So how it happens is that there are the cell receptors called T cell receptors those are on the T. Cell. And they actually have to directly contact the antigen presenting cell that has the antigen. And so they directly interact with that. And again to help activate it which means that they have to be as diverse as the antigens are. So T cell receptors also undergo the V. D. J. Recombination that we talked about in antibodies um and the antibody topic in order to have enough diversity to recognize that diversity advantages. So once they so they need to be activated. So T cells are activated in the thymus which is the special organ in your neck and throat region and the thymus um in the thymus you get naive T cells naive T cells these are t cells that aren't activated aren't responding to an and again they contact these antigen presenting cells that are complex with antigens in the thymus. And in order to be activated the T cell has to interact with three components. They have to interact with the antigen on the A. P. C. They have to interact with specific co stimulatory molecules to help stimulate that activation. And they have to interact with cell to cell adhesion molecules between themselves and that antigen presenting cell. And if all three of these don't contact then what happens is that the T cell is saying oh I'm being activated when I shouldn't be. So it either in activates itself or it dies. So that's kind of like a regulatory thing to make sure that T cells aren't activated just kind of willy nilly they're activated when they meet these certain criteria. So here we have an example we have an antigen presenting cell here it's taken up an antigen, It's chopped it up into these little pieces and it's presented on the surface through this complex called an MHC molecule. Now I haven't talked about this yet, there's an entire topic on these molecules that I really suggest you check out because there are some intricacies that you're gonna need to know about these molecules but that's not this topic. This topic says this antigen presenting cell uses these MHC to present to a T cell. This is a T helper cells which I'll talk about in a second and then this can becomes activated and goes on to do a lot of things that activates other cells activate cells activates different cells that will kill itself. But there are many types of T cells that can do different things. So let's talk about those. So there are three types of T cells. The first is going to be the side of toxic T cells which you'll see written as T. C. These directly kill cells that have an intracellular pathogen. So the so seidel toxic T cells come in contact with some kind of antigen presenting cell with an antigen presented on the surface and then it induces the cell to kill itself. A couple ways to do this. Not going to go over all of them. But just a few that your book may mention is the performance and this means that the T cells will release this and that forms pores and that antigen presenting cell which will kill the cell and kill the pathogen. Or it can use this lichen called fast which we briefly talked about before but not a lot. But this Lagan this ligand um will interact with that antigen presenting cell and stimulate apoptosis which will kill the cell and kill the pathogen. So that's if you decide to talk to T cells kill the cell and kill the pathogen helper T cells are a little different written as th and these are responsible for stimulating the responses of other cells So they can once activated themselves they go on to activate B cells. Macrophages dendritic cells whatever And th cells can also release um co stimulatory molecules that can activate distant cells. So helper T cells are kind of helping along the immune response by activating a bunch of cells. And then you have the last type of T cells regulatory T cells and these are responsible for suppressing the activity of other T cells. And you say whoa why would I want to suppress the activity of T cells? Well eventually you know this you're not going to be having this strong immune response. Eventually you're going to need to suppress it and calm it down. So regulatory T cells are really important for that but they're also important for making sure that t cells that are self reactive which means that they're you know responding to everything, even things that aren't foreign um they're responding to themselves that those get suppressed as well. So these are really important to sell in regulating the activity of the immune response. So here we have an example. So we have a T cell here and T. Cell and we have an antigen presenting cell. You can read this text if you want to. It'll probably help you out but I'm not gonna read it here. So the antigen presenting cell has this auntie jen here and you can see it takes in this and again it's an intracellular antigens. It um you know chops it up and eventually presents that Auntie jen, A pc is gonna present that and again to the T cell. This T cell then becomes activated. Now there are some T cells that are helper t cells that go on to activate other cells. There are some T cells that are cited toxic cells that go on to kill the cells and then um other cells or regulatory cells that will regulate this this reaction. So that is an overview of T cells. So you don't think there's anything else? Oh no, there is. Almost forgot. Okay, there are two important accessory molecules that all t cells contain this will be really sure. I know some of you are like, oh yeah, videos over, sorry. So these two accessory molecules are called C. D. A. N. C. D four, C. D. A is found on site of toxic and cd four is found on helper and regulatory cells. These are super important because they're only found on these certain T cells and they only buying certain MHC molecules which I'm going to talk about in other videos. So this is Class one and this is Class two. Um now I know we haven't talked about those yet and you're like, what are these MHC molecules but you're gonna need to know all of these facts here. C. B. A. Which T cells which Class one, even though we haven't talked about the MHC there with me. It's going to be in the next video. Um And then you'll know more about what I'm talking about when I do these Class one and Class two MHC molecules and these are required. If these aren't here they're not it's not gonna work. So the accessory receptors of C. D. A. N C. D four stabilize the binding and they help activate the T cells. So here we have an example, we have a T. Helper cell and it's going to be activating a B cell. So you can see that it has the C. D four here, it has the T cell receptor here. You can see the MHC Class two molecule here um And this is going to lead to the activation of this B cell and it is super important. So now that's definitely the end. So let's now turn the page.
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Problem
ProblemCytotoxic T cells are responsible for which of the following responses?
A
Suppressing other T cells
B
Activating B cells
C
Killing other infected cells
D
Activating antigen presenting cells
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Problem
ProblemWhich of the following cell types is responsible for suppressing the activity of T cells?
A
Antigen presenting cells
B
Cytotoxic T cells
C
Helper T cells
D
Regulatory T cells
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Problem
ProblemCD4 molecules are found on which of the following cell types?
A
Antigen presenting cells
B
Cytotoxic T cells
C
Helper T cells
D
Regulatory T cells