T Cells: Videos & Practice Problems

T cells are crucial components of the adaptive immune system, activated by antigen presenting cells (APCs) like dendritic cells. Activation requires interaction with antigens, co-stimulatory molecules, and adhesion molecules. There are three main types of T cells: cytotoxic T cells (kill infected cells), helper T cells (activate other immune cells), and regulatory T cells (suppress immune responses). Accessory molecules CD8 and CD4 are essential for T cell function, binding to MHC class I and II molecules, respectively, facilitating effective immune responses against pathogens.

T Cells

Hi. In this video, I'm going to be talking about T cells. So, T cells are a main immune cell, part of the adaptive immune system that is used to target and kill pathogens. First, they have to be activated. They're not normally activated. The type of cell that activates a T cell is called an antigen presenting cell. What this does is exactly what it sounds like. It takes a pathogen, chops it up, degrades it a little, and takes those little antigens, presents them to T cells, and that results in activation. A good example of this are dendritic cells, which are part of the innate immune system, but other cells can do this as well.

How it happens is that there are 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. So they directly interact with that antigen to help activate it, which means that they have to be as diverse as the antigens are. T cell receptors also undergo the VDJ recombination that we talked about in antibodies, in the antibody topic in order to have enough diversity to recognize the diversity of antigens. Once they need to be activated, T cells are activated in the thymus, which is the special organ in your neck and throat region.

In the thymus, you get naive T cells, naive T cells. These are T cells that aren't activated, aren't responding to an antigen. They contact these antigen presenting cells that are complex with antigens in the thymus. In order to be activated, the T cell has to interact with three components. They have to interact with the antigen on the APC, 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. If all three of these don't contact, then what happens is that the T cell is saying, oh, I'm being inactivated when I shouldn't be. So it either inactivates itself or it dies. That's kind of like a regulatory thing to make sure that T cells aren't activated just willy-nilly. They are activated when they meet these certain criteria.

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 going to 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 cell, which I'll talk about in a second. And then this becomes activated and goes on to do a lot of things.

It activates other cells, different cells that will kill the cell, but there are many types of T cells that can do different things. So let's talk about those. There are three types of T cells. The first is going to be the cytotoxic T cells, which you'll see written as TC. These directly kill cells that have an intracellular pathogen. So cytotoxic 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. Couple ways to do this, not going to go over all of them, but just a few that your book may mention are the perforin, and this means that the T cells will release this, and that forms pores in that antigen presenting cell, which will kill the cell and kill the pathogen. Or it can use this ligand called FAS, which we briefly talked about before, but not a lot. But this ligand will interact with that antigen presenting cells to kill the cell and kill the pathogen. So that's if cytotoxic 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. TH cells can also release 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.

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 can 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 are, you know, responding to everything, even things that aren't foreign, they're responding to themselves, that those get suppressed as well. So these are really important T cells in regulating the activity of the immune response.

So here we have an example. We have a T cell here and a T cell, and we have an antigen presenting cell. You can read this text if you want to, it will probably help you out, but I'm not going to read it here. So the antigen presenting cell has this antigen here, and you can see it takes in this antigen, it's an intracellular antigen. It, you know, chops it up and eventually presents that antigen, APC. It's going to present that antigen 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 cytotoxic cells that go on to kill this cell. And then, other cells are regulatory cells that will regulate this reaction. So that is an overview of T cells. I 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 short. I know some of you are like, oh, yay, video is over. Sorry. So, these two accessory molecules are called CD8 and CD4. CD8 is found on cytotoxic and CD4 is found on helper and regulatory cells. These are super important because they are only found on certain T cells, and they only bind certain MHC molecules which I'm going to talk about in other videos. So this is class 1 and this is class 2. Now I know we haven't talked about those yet, and you're like, what are these MHC molecules? But you're going to need to know all of these facts here. CD8, which T cells, which class 1? Even though we haven't talked about the MHC, bear with me, it's going to be in the next video, and then you'll know more about what I'm talking about when I do these class 1 and class 2 MHC molecules. And these are required, if these aren't here they're not, it's not going to work. So, the accessory receptors of CD8 and CD4 stabilize the binding and they help activate the T cells. So, here we have an example. We have a T helper cell and it is going to be activating a B cell. So you can see that it has the CD4 here, it has the T cell receptor here, you can see the MHC class 2 molecule here, and this is going to lead to the activation of this B cell, and it's super important. So now that's definitely the end, so let's now turn the page.

Cytotoxic 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

Answer: (C)

Which 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

Answer: (D)

CD4 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

Okay. So this question says CD4 molecules are found on which of the following cell types? Antigen-presenting cells, cytotoxic T cells, helper T cells, and regulatory T cells. So what do you think? So hopefully, you're a bit confused because you may start realizing there's actually two choices here, and that's okay. So CD4 molecules are found on helper T cells and regulatory T cells, so c and d are both the correct answer. Cytotoxic T cells have CD8 molecules, and the antigen-presenting cells have none of those. So, the answer to this question is both c and d. With that, let's move on.

Here’s what students ask on this topic:

What are the main types of T cells and their functions?

There are three main types of T cells: cytotoxic T cells (Tc), helper T cells (Th), and regulatory T cells (Treg). Cytotoxic T cells directly kill infected cells by recognizing antigens presented on MHC class I molecules. They release perforin and granzymes to induce apoptosis in the target cell. Helper T cells, which express CD4, activate other immune cells like B cells, macrophages, and dendritic cells by releasing cytokines and interacting with MHC class II molecules. Regulatory T cells suppress the immune response to prevent overactivation and autoimmunity, ensuring that the immune system does not attack the body's own tissues.

How are T cells activated?

T cells are activated by antigen presenting cells (APCs) such as dendritic cells. The activation process requires three key interactions: the T cell receptor (TCR) must recognize and bind to the antigen-MHC complex on the APC, co-stimulatory molecules on the APC must engage with receptors on the T cell, and cell adhesion molecules must stabilize the interaction. If all three criteria are met, the T cell becomes activated. This process ensures that T cells are only activated in the presence of a genuine threat, preventing unnecessary immune responses.

What role do CD8 and CD4 molecules play in T cell function?

CD8 and CD4 are accessory molecules found on T cells that are crucial for their function. CD8 is found on cytotoxic T cells and binds to MHC class I molecules, helping these T cells recognize and kill infected cells. CD4 is found on helper and regulatory T cells and binds to MHC class II molecules, facilitating the activation of these T cells. These interactions are essential for the stabilization and activation of T cells, ensuring a precise and effective immune response.

What is the role of the thymus in T cell development?

The thymus is a specialized organ located in the neck and throat region, crucial for T cell development. In the thymus, immature T cells, known as thymocytes, undergo a selection process to become mature, naive T cells. This process involves positive selection, where T cells that can recognize self-MHC molecules are selected, and negative selection, where T cells that strongly react to self-antigens are eliminated. This ensures that the mature T cells are capable of recognizing foreign antigens while being tolerant to the body's own tissues.

How do antigen presenting cells (APCs) activate T cells?

Antigen presenting cells (APCs) activate T cells by processing and presenting antigens on their surface using MHC molecules. APCs, such as dendritic cells, ingest pathogens, degrade them, and present the resulting antigen fragments on MHC class I or II molecules. T cells recognize these antigen-MHC complexes through their T cell receptors (TCRs). Additionally, co-stimulatory signals and cell adhesion molecules are required to fully activate the T cells. This multi-step process ensures that T cells are only activated in the presence of a genuine threat, preventing unnecessary immune responses.