Introduction to T Lymphocytes - Online Tutor, Practice Problems & Exam Prep

T lymphocytes, or T cells, are crucial for adaptive immunity, specifically in targeting intracellular pathogens. They develop in the thymus and migrate to secondary lymphoid organs. T cells can be activated by antigen-presenting cells, leading to differentiation into effector T cells, which respond immediately, or memory T cells, which provide long-term immunity. Cytotoxic T cells (CD8) induce apoptosis in infected cells, while helper T cells (CD4) activate other immune components. Understanding these functions is essential for grasping immune responses and the role of T cells in health and disease.

Introduction to T Lymphocytes

In this video, we're going to begin our introduction to T lymphocytes or T cells. 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 is a part of adaptive immunity. Cell-mediated immunity targets and destroys intracellular pathogens, or disease-causing agents that have made their way inside of a host cell. It does this by using, once again, T cells or T lymphocytes. We are going to continue to introduce and talk about T cells and T lymphocytes as we move forward. What you'll notice here is that we're showing you the primary lymphoid organs here, and recall that T cells are developed in the thymus. 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, lymph nodes, or the spleen.

Really, what we're going to learn moving forward is that there are two major types of T cells. There are cytotoxic T cells or TC cells, and there are also helper T cells or TH cells. As we move forward in our course, we will differentiate between these two types of T cells. Both the cytotoxic T cell and the helper T cell need to be activated by an antigen-presenting cell such as a dendritic cell. Here, we show a dendritic cell presenting antigens on its surface so that they can activate these T cells. Upon activation of the T cells, these T cells will begin to divide and differentiate into either memory T cells—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. In addition to generating these memory cells, they also differentiate into effector cells as well, effector helper T cells, and also effector cytotoxic T cells.

What you'll notice is that it's these effector T cells that can go on to carry out the immune functions. The memory T cells are stored for generating an immune response to a secondary infection, a potential secondary infection. 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. Everything that we've discussed here in this video about the T cells will be broken down and explained in more details as we move forward. This concludes our brief introduction to T lymphocytes. 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 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 again 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 lessons 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. This variable region is the part that will directly bind to the presented antigen. 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 in this variable region accounts for the many different types of antigens that different TCRs can bind. 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 toward 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 a region that remains relatively constant in terms of its amino acid sequence. This constant region is the region that's closest to the cell surface and is very constant in its amino acid sequence. Take a look at our image below, where you can get a better understanding of T cell receptors. On the left-hand side, we are showing you a T cell. Recall that these T cells are going to have thousands of identical T cell receptors. Notice that here we are showing you these T cell receptors that these T cells can have. If we zoom in to this particular T cell receptor, you can see, that 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. 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. Also, 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. Constant & Variable Regions.
• D. Beta chain.
• 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, they 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, these cells 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. This helps to create a stronger immune response when you are exposed to one of these antigens a second time.

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 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. And so 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 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. In addition to differentiating into effector cytotoxic T cells, the cytotoxic 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.

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. 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 activated. We have T cell activation, and again that leads to the differentiation and the proliferation of these cells. It can differentiate into 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. 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. And so, we need to recall from our previous lesson videos that there are two types of effector and memory T cells that are responsible for generating an immune response to antigens. And those two main types of T cells are once again the cytotoxic T cell and the helper T cell. Now, the cytotoxic T cell is commonly abbreviated as T cell or CD8 cell. And the CD8 is specifically referring to a specific protein that's on the surface of these cytotoxic T cells. And so cytotoxic T cell, T C cell, and CD8 cell are all referring to the same types of cells. And these cytotoxic T cells, they're mainly responsible for releasing chemicals that induce apoptosis of host cells that are infected with intracellular pathogens. So, once again, these T cells are targeting 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 going to be commonly abbreviated as TH or CD4 cells. And again, CD4 is just the name of a protein that's found on the surface of these helper T cells. And, helper T cells, T H cell, and CD four cell are all referring to the same type of cell. Now once again, the helper T cells are different than 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. And so, they are going to produce cytokines to help stimulate and activate other immune cells. For example, helping to stimulate and activate B cells. And again, we'll get to talk more about B cells later in our course after we've finished talking about the T cells. And so again, the cytotoxic T cell can be abbreviated as CD8 cell, and the helper T cell can be abbreviated as CD4 cell. And that's something that's very important for you to remember. And so again, CD here actually stands for cluster of differentiation, and these are specific markers, protein markers that are on the surface of these specific types of cells. And it's one of the ways that scientists can differentiate between the two types of T cells. And so, these CD markers are clusters of surface proteins on T cells that again can be used to differentiate cytotoxic T cells and helper T cells. So again, the CD markers, the cytotoxic T cells are CD8 and the helper T cells are CD4. And it's these CD markers that can actually dictate or help to dictate the interactions and the functions of the cytotoxic and helper T cells. And so if we take a look at our image down below, we can further differentiate between the cytotoxic and the helper T cell. And so, on the left-hand side of our image, what we're showing you is the cytotoxic T cell. And what you'll notice is that our cytotoxic T cell has the CD8 markers on its surface. And again, these CD8 markers are going to allow for scientists to basically differentiate the cytotoxic T cell from the helper T cell which we have on the right. Now, the cytotoxic T cell we're showing here also, as the grim reaper here because what it does is it can induce apoptosis in host cells that have been affected by an intracellular pathogen. So for example, over here we have another host cell that is infected, and it is infected with a virus as you can see right here. And so because this host cell is infected, this cytotoxic T cell can come, and it can induce apoptosis of this infected host cell. And that ultimately helps to protect the entire human body and defend ourselves against these intracellular pathogens. Now again, we'll get to talk a lot more details about the functions of cytotoxic T cells as we move forward in our course. Now on the right over here, what we're showing you are the helper T cells. And the helper T cells are also known as TH cells or CD four cells, And that's because they have CD4 markers, or proteins on the surface of these cells. And, these helper T cells, they are different than the cytotoxic T cells. They have a different function. Instead of inducing apoptosis, these helper T cells are going to help activate other cells, so they can activate other reinforcements, if you will. And so, notice over here what we're showing you are a bunch of other immune system cells that can become activated, and they become activated by the help of the CD4 cells. And so, the activated immune cells are going to be capable of generating immune responses and improved immune responses as well. And again, we'll get to talk more details about the functions of helper T cells as we move forward in our course. And notice that we're showing you the helper T cell here as a little like a crosswalk person that helps people cross the roads and things like that because again, it's going to be helping other immune cells to perform their functions better. And so this here concludes our brief introduction to the cytotoxic T cell and helper T cells, and again we'll be able to apply these concepts as we move forward and learn more about them as well. So 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 response. Cytotoxic T cells, also known as CD8 cells, are primarily responsible for inducing apoptosis in host cells infected with intracellular pathogens, such as viruses. They achieve this by releasing chemicals that trigger cell death, thereby eliminating the infected cells. 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 macrophages, enhancing the overall immune response. This activation is crucial for a coordinated and effective defense against pathogens.

How do T cell receptors (TCRs) recognize antigens?

T cell receptors (TCRs) recognize antigens through a highly specific interaction with antigen-presenting cells (APCs). Each TCR is composed of two polypeptide chains, an alpha chain and a beta chain, linked by disulfide bonds. These chains have variable regions that differ between TCRs, allowing them to bind to a wide variety of antigens. However, TCRs can only bind to antigens that are presented on the surface of APCs, such as dendritic cells, in the context of major histocompatibility complex (MHC) molecules. This specificity ensures that T cells are activated only in the presence of a specific antigen, leading to a targeted immune response.

What is the difference between naive, effector, and memory T cells?

Naive T cells are inactive T cells that have not yet encountered their specific antigen. Upon encountering an antigen presented by an antigen-presenting cell, naive T cells become activated and differentiate into effector and memory T cells. Effector T cells are short-lived and provide an immediate immune response to the current infection. They include cytotoxic T cells that kill infected cells and helper T cells that activate other immune cells. Memory T cells, on the other hand, are long-lived and persist in the body for years, providing a rapid and robust response to subsequent exposures to the same antigen, thereby offering long-term immunity.

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

T cells develop in the thymus, a primary lymphoid organ. Once they are fully developed, they migrate to secondary lymphoid organs, such as the tonsils, lymph nodes, and spleen. In these secondary lymphoid organs, T cells encounter antigen-presenting cells that present specific antigens, leading to their activation. This migration and subsequent activation are crucial for the T cells to perform their roles in adaptive immunity, targeting and destroying intracellular pathogens and coordinating the immune response.

How do memory T cells contribute to long-term immunity?

Memory T cells contribute to long-term immunity by persisting in the body for many years, sometimes even the lifetime of the individual. These cells are generated after the initial activation of naive T cells during the first encounter with a specific antigen. Unlike effector T cells, memory T cells do not respond to the initial infection but are primed to respond rapidly and robustly to subsequent exposures to the same antigen. This quick response helps to prevent reinfection and provides a stronger and more efficient immune defense upon re-exposure to the pathogen.