Introduction to T Lymphocytes: Study with Video Lessons, Practice Problems & Examples

T lymphocytes, or T cells, are crucial for cell-mediated immunity, targeting intracellular pathogens. They develop in the thymus and migrate to secondary lymphoid organs. T cells include cytotoxic T cells (CD8) that induce apoptosis in infected cells and helper T cells (CD4) that activate other immune cells. Naive T cells become activated upon encountering antigens presented by antigen-presenting cells, differentiating into effector or memory T cells. Memory T cells provide a faster response to future infections, enhancing immune efficiency.

Cell-Mediated Immunity

In this video, we're going to begin our introduction to T lymphocytes or T cells. And so first, we need to recall from some of our previous lesson videos that T lymphocytes or T cells play an important role in cell-mediated immunity, which recall is a part of adaptive immunity. And so cell-mediated immunity is going to be targeting and destroying intracellular pathogens, or disease-causing agents that have made their way inside of a host cell. And it does this by using, once again, T cells or T lymphocytes. And so down below, what we're going to do is focus on this image, but we're going to continue to introduce and talk about T cells and T lymphocytes as we move forward. And so what you'll notice here is we're showing you the primary lymphoid organs here, and recall that T cells are going to be developed in the thymus. And after these T cells are fully developed, the naive forms of the T cells or the inactive forms of the T cells will migrate to the secondary lymphoid organs, like tonsils or lymph nodes or the spleen. And so really what we're going to learn moving forward is that there are 2 major types of T cells. There are cytotoxic T cells or TC cells, and then there are also helper T cells or TH cells. And as we move forward in our course, we will differentiate between these two types of T cells. Now, here we're showing you that both the cytotoxic T cell and the helper T cell need to be activated by an antigen-presenting cell such as a dendritic cell. And so here we're showing you a dendritic cell presenting antigens on its surface so that they can activate these T cells. And upon activation of the T cells, these T cells will begin to divide and differentiate into either memory T cells, so notice that there are both memory cytotoxic T cells, memory TC cells, but there are also memory TH cells as well or memory helper T cells. And in addition to generating these memory cells, they also will differentiate into effector cells as well, effector helper T cell, and also effector cytotoxic T cell. And so, what you'll notice is that it's these effector T cells that can go on to carry out the immune functions. And the memory T cells are just stored for generating an immune response to a secondary infection, a potential secondary infection. Now notice that the B cells are being shown over here on the right-hand side, but we're going to talk about the B cells later in our course after we finish talking about the T cells. And so everything that we've discussed here in this video about the T cells we're going to break down and explain in more details as we move forward. And so this here concludes our brief introduction to T lymphocytes. And again, we'll continue learning about them as we go forward. So I'll see you all in our next video.

Which of the following answers is a major difference between cell-mediated and humoral immunity?

• A. Cell-mediated immunity utilizes T cells while humoral immunity utilizes B cells to respond to infection.
• B. Cell-mediated immunity involves antigens presented on APCs while humoral immunity involves free antigens.
• C. Cell-mediated immunity responds to antigens outside of cells. Humoral immunity responds to antigens within a cell.
• D. A and B.
• E. B and C.
• F. All of the above are major differences between cell-mediated and humoral immunity.

T-Cell Receptors

In this video, we're going to talk more details about T cell receptors. First, we need to recall from some of our previous lesson videos that T cells are immune system cells that play a huge role in adaptive cell mediated immunity. Recall that these T cells develop in the thymus, and these T cells have thousands of identical T cell receptors that are embedded in their membranes. Now these T cell receptors are commonly abbreviated as TCRs. These T cell receptors or TCRs are receptors that allow T cells to recognize and attack very specific intracellular pathogens, or pathogens that are on the inside of a host cell. Also recall from some of our previous lesson videos that these T cell receptors are not capable of binding free antigens. These T cell receptors can only bind to presented antigens or antigens that are presented on the surface of antigen-presenting cells. In terms of the structure of these TCRs, each TCR consists of 2 polypeptide chains, an alpha chain and a beta chain. These 2 polypeptide chains are linked via special types of covalent bonds known as disulfide bonds. Each of these polypeptide chains contains 2 different regions. The first region that each of them contains is a variable region, and the second region that each of them contains is a constant region. The variable region of each of the polypeptide chains, as its name implies, is a region of the polypeptide chain that varies between different TCRs. Different TCRs will have different variable regions and this variable region is the region that will directly bind to the presented antigen. Once again, the TCRs are only capable of binding to presented antigens. They are not capable of binding to free antigens. The amino acid sequence variation that's in this variable region accounts for the many different types of antigens that different TCRs can bind. Different TCRs are capable of binding to different antigens. But once again, a single T cell will have thousands of identical T cell receptors, which means that a single T cell is only capable of generating an immune response towards one very specific antigen. The constant region is the second region that each of these polypeptide chains contains. The constant region as its name implies is going to be a region that remains relatively constant, in terms of its amino acid sequence. It is the region that's closest to the cell surface and it is going to be very constant in its amino acid sequence. If we take a look at our image down below, we can get a better understanding of these T cell receptors. Notice on the left-hand side over here we're showing you a T cell. Again, recall that these T cells are going to have thousands of identical T cell receptors. Notice that here we're showing you these T cell receptors that these T cells can have. Notice that if we zoom into this particular T cell receptor, we can see that is what this image is showing, a zoom in of this particular T cell receptor. When we look at the entire T cell receptor, you'll notice that it has those 2 polypeptide chains. It has this alpha chain over here on the left and then it has this beta chain over here on the right. The two chains are linked via disulfide bonds. Now notice that each of these chains contains a variable region, which is here at the top. This variable region that you see right here again is going to vary from one TCR to another TCR, vary between different TCRs and it's going to allow for the binding to a presented antigen. Down below, in this green region what we have is the constant region of these polypeptide chains. The constant region is going to remain relatively constant from one TCR to a different TCR. This here concludes our brief introduction to these T cell receptors. Once again, we'll be able to get some practice applying these concepts and learn more about T lymphocytes as we move forward in our course. I'll see you all in our next video.

Which of the following is NOT a component of TCRs?

• A. Alpha chain.
• B. CD3 complex.
• C. Beta chain.
• D. Constant & Variable Regions.
• E. None of the above.

What is the importance of the variable region of T-cell receptors?

• A. The variable region is the closest region to the cell’s surface and stabilizes the receptor in the plasma membrane.
• B. The variable region directly binds to antigens.
• C. The variable region varies in its amino acid sequence allowing different TCRs to bind different antigens.
• D. The variable region directly binds to antibodies.
• E. A and B.
• F. B and C.
• G. C and D.

Naive T Cells Become Effector & Memory T Cells

In this video, we're going to introduce naive T cells, effector T cells, and memory T cells. And we'll also talk about how these naive T cells can actually become effector and memory T cells. Now it is important to note that before a developed T cell encounters its presented antigen that it's supposed to generate an immune response to, that T cell exists in an inactive form that we refer to as a naive T cell. A naive T cell is a T cell that is inactive because it has not yet encountered a presented antigen. Before the T cell can actually generate an immune response, the naive T cell must become activated. When naive T cells encounter a presented antigen on an antigen-presenting cell such as, for example, a dendritic cell, then that naive T cell can become activated. Activated T cells are capable of doing two things. They're capable of proliferating or multiplying to create more identical clones of itself, and they're also capable of differentiating or changing their phenotype to become a slightly different cell type. When they differentiate to change their phenotype, they can differentiate into either an effector T cell or they could differentiate into a memory T cell. The effector T cells are going to be short-lived cells. These effector T cells that are short-lived, generate an immediate immune response to the first infection upon encountering the antigen for the first time. Now the memory T cells, on the other hand, are long-lived cells that can survive for many years, perhaps even the lifespan of the human being. These memory T cells that are long-lived also remember the antigen. By remembering the antigen, what that means is that they do not respond to the first infection. However, they will generate a very fast response to any future infection, such as a secondary infection. That helps to create a stronger immune response when you are exposed to one of these antigens a second time. We'll get to talk more about these details here as we move forward in our course. It is also important to note that there are two main types of effector T cells and two main types of memory T cells, and those are going to be the cytotoxic and helper T cells. That means that there are effector cytotoxic effector T cells, cytotoxic memory T cells, and there are also helper effector T cells and helper memory T cells. If we take a look at our image down below, we can get a better understanding of all of these T cells. Notice on the left-hand side over here, we're focusing on the cytotoxic T cells. Remember that the cytotoxic T cells have an effector cytotoxic T cell, and there's also memory cytotoxic T cells. Notice that these T cells originally start off in their naive forms. Here we have a naive T cell and recall that the naive T cell is going to be inactive because it has not yet encountered its presented antigen. However, when the naive T cell is introduced to its presented antigen by an antigen presenting cell such as a dendritic cell. This cell over here is our antigen-presenting cell presenting an antigen to this naive T cell. When the naive T cell recognizes a presented antigen, then that can lead to T cell activation, which is what we see right here. This T cell activation will allow the T cell to proliferate, divide, and differentiate into either an effector cytotoxic T cell. So this would be a cytotoxic, an effector cytotoxic T cell being represented right here. Notice that we're showing this cytotoxic T cell somewhat like the grim reaper here because it's going to have some immune responses that are similar to a grim reaper. We'll be able to talk about that later as we move forward in our course. Again, in addition to differentiating into effector cytotoxic T cells, the cytotoxic the activation of the naive cytotoxic T cell could also lead to the differentiation into a memory cytotoxic T cell, which is what we have over here. The memory cytotoxic T cell does not respond to the very first infection. Instead, it's a long-lived cell that is capable of responding faster to future infections. Notice that this memory cytotoxic T cell here is saying I will remember this antigen if we encounter it again in the future. Now over here on the right-hand side, we have an analogous image, but instead of cytotoxic T cells, these are for helper T cells. Again, the helper T cells also have an effector helper T cell and a memory helper T cell. Again, it originally starts off as a naive T cell. But upon recognizing and being presented its antigen by an antigen presenting cell, like a dendritic cell, the naive T cell, which is inactive, can become a helper T cell, which is the effector T cell, and, or it could differentiate into a memory T cell, which again, is going to generate a faster response in future infections. The helper T cell is going to, as its name implies, help activate other components of the immune system. We'll be able to talk more about the actual immune responses of these T cells as we move forward in our course. But for now, this here has concluded our lesson on naive T cells and how these naive T cells can become activated to differentiate into either effector T cells or memory T cells, and how there are also cytotoxic and helper versions of each of these T cells. This here concludes this lesson, and we'll be able to get some practice applying this as we move forward. So I'll see you all in our next video.

A naive lymphocyte:

• A. Has encountered an antigen with its receptor but has not yet produced antibodies.
• B. Has not yet encountered an antigen recognized by its receptors but is producing antibodies.
• C. Has encountered an antigen and has undergone apoptosis.
• D. Has fully developed antigen receptors but has not yet encountered an antigen.
• E. Has produced antibodies despite not having developed receptors.

Which of the following statements about effector T cells is NOT true?

• A. Effector T cells are long-lived cells that allow the immune system to respond quickly to subsequent infections.
• B. Effector T cells can take two forms: Cytotoxic T Cells & Helper T Cells.
• C. Effector T cells trigger an immediate immune response to a novel infection.
• D. Effector T cells are short-lived immune cells.
• E. All of the above statements about effector T cells are true.

Cytotoxic T Cells vs. Helper T Cells

In this video, we're going to further differentiate between cytotoxic T cells and helper T cells. We need to recall from our previous lesson videos that there are 2 types of effector and memory T cells responsible for generating an immune response to antigens. Those two main types of T cells are once again the cytotoxic T cell and the helper T cell. The cytotoxic T cell is commonly abbreviated as T cell or CD8 cell. The CD8 specifically refers to a protein on the surface of these cytotoxic T cells. Cytotoxic T cell, TC cell, and CD8 cell all refer to the same types of cell. These cytotoxic T cells are mainly responsible for releasing chemicals that induce apoptosis of host cells infected with intracellular pathogens.

Now, the helper T cell, on the other hand, is going to have a different function than the cytotoxic T cell. The helper T cell is commonly abbreviated as TH or CD4 cells. CD4 is the name of a protein found on the surface of these helper T cells. Helper T cells, TH cell, and CD4 cell all refer to the same type of cell. The helper T cells are different from cytotoxic T cells. They do not have the same functions. The helper T cell, as its name implies, is going to help other cells become activated. They produce cytokines to help stimulate and activate other immune cells, such as stimulating and activating B cells. We will discuss more about B cells later in our course after we have finished talking about T cells.

CD here stands for cluster of differentiation, and these are specific markers, protein markers on the surface of these specific types of cells. It is one of the ways scientists can differentiate between the two types of T cells. These CD markers are clusters of surface proteins on T cells can be used to differentiate cytotoxic T cells and helper T cells. The CD markers, the cytotoxic T cells are CD8, and the helper T cells are CD4. These CD markers can dictate or help to dictate the interactions and the functions of the cytotoxic and helper T cells.

If we look at our image down below, we can further differentiate between the cytotoxic and the helper T cells. On the left-hand side of our image, we show you the cytotoxic T cell. You will notice that our cytotoxic T cell has the CD8 markers on its surface. These CD8 markers allow scientists to differentiate the cytotoxic T cell from the helper T cell, which we have on the right. The cytotoxic T cell we are showing here also, as the grim reaper, because it can induce apoptosis in host cells that have been affected by an intracellular pathogen. For example, over here, we have another host cell that is infected with a virus. Because this host cell is infected, the cytotoxic T cell can induce apoptosis of this infected host cell. That ultimately helps to protect the entire human body and defend ourselves against these intracellular pathogens.

On the right, we show you the helper T cells, also known as TH cells or CD4 cells because they have CD4 markers or proteins on the surface of these cells. These helper T cells are different from the cytotoxic T cells; they have a different function. Instead of inducing apoptosis, these helper T cells help activate other cells so they can activate other reinforcements, if you will. Notice here we show you a bunch of other immune system cells that can become activated by the help of the CD4 cells. The activated immune cells are going to be capable of generating immune responses and improving immune responses as well.

We will discuss more details about the functions of helper T cells as we move forward in our course. Notice that we're showing you the helper T cell here as a little person, like a crosswalk assistant that helps people cross the roads because again it's going to help other immune cells to perform their functions better. This concludes our brief introduction to the cytotoxic T cell and helper T cells, and we will be able to apply these concepts as we move forward and learn more about them as well. I'll see you all in our next video.

CD markers differentiate the two types of effector T cells. Cytotoxic T cells have _________ markers while helper T cells have _________ markers.

• A. CD3; CD4.
• B. CD4; CD5.
• C. CD8; CD6.
• D. CD8; CD4.

Which of the following statements about cytotoxic T cells is true?

• A. When cytotoxic T cells encounter an antigen, they produce cytokines to stimulate other immune cells.
• B. When cytotoxic T cells encounter “presented” antigens, they send signals triggering apoptosis in the infected cell.
• C. When cytotoxic T cells encounter an antigen, they recruit B cells to produce antibodies.
• D. Cytotoxic t cells only recognize and respond to “free” antigens.

Here’s what students ask on this topic:

What are the main functions of cytotoxic T cells (CD8) and helper T cells (CD4)?

Cytotoxic T cells (CD8) and helper T cells (CD4) have distinct roles in the immune system. Cytotoxic T cells, also known as CD8 cells, are primarily responsible for inducing apoptosis in host cells that are infected with intracellular pathogens, such as viruses. They release chemicals that trigger the death of these infected cells, thereby preventing the spread of the pathogen. On the other hand, helper T cells, or CD4 cells, do not directly kill infected cells. Instead, they produce cytokines that help activate and stimulate other immune cells, such as B cells and cytotoxic T cells, enhancing the overall immune response. This activation is crucial for a coordinated and effective immune defense.

How do naive T cells become activated and what do they differentiate into?

Naive T cells are inactive T cells that have not yet encountered their specific antigen. They become activated when they encounter an antigen presented by an antigen-presenting cell (APC), such as a dendritic cell. Upon activation, naive T cells proliferate, creating identical clones of themselves, and differentiate into either effector T cells or memory T cells. Effector T cells are short-lived and generate an immediate immune response to the first infection. Memory T cells, on the other hand, are long-lived and can survive for many years, providing a faster and more robust response to future infections by the same antigen.

What is the structure of T cell receptors (TCRs) and how do they function?

T cell receptors (TCRs) are composed of two polypeptide chains, an alpha chain and a beta chain, linked by disulfide bonds. Each chain has a variable region and a constant region. The variable region, which varies between different TCRs, is responsible for binding to presented antigens on the surface of antigen-presenting cells. The constant region remains relatively unchanged and is closest to the cell surface. TCRs allow T cells to recognize and respond to specific intracellular pathogens. However, TCRs cannot bind free antigens; they can only bind antigens that are presented by APCs.

What roles do memory T cells play in the immune system?

Memory T cells are long-lived cells that play a crucial role in the immune system by providing a faster and more efficient response to future infections by the same antigen. Unlike effector T cells, memory T cells do not respond to the initial infection. Instead, they remain in the body for many years, sometimes for the lifespan of the individual. When the same antigen is encountered again, memory T cells quickly recognize it and mount a rapid and robust immune response, thereby enhancing the body's ability to fight off repeated infections more effectively.

Where do T cells develop and where do they migrate after development?

T cells develop in the thymus, a primary lymphoid organ. After they are fully developed, the naive forms of T cells, which are inactive, migrate to secondary lymphoid organs such as the tonsils, lymph nodes, and spleen. In these secondary lymphoid organs, T cells encounter antigens presented by antigen-presenting cells, which leads to their activation and differentiation into effector and memory T cells. This migration and subsequent activation are crucial for the T cells to perform their roles in adaptive immunity effectively.