Activation of T Lymphocytes - Online Tutor, Practice Problems & Exam Prep

T lymphocytes are activated by antigen-presenting cells (APCs) like dendritic cells, which present harmful antigens using MHC class 1 or class 2. This triggers the production of co-stimulatory molecules, leading to T cell activation, proliferation, and differentiation into effector or memory T cells. Conversely, when harmless antigens are presented, no co-stimulatory signals are produced, resulting in T cells becoming anergic and undergoing apoptosis. This mechanism ensures the immune system targets only dangerous antigens, maintaining immune tolerance to harmless substances.

T-Cell Activation

In this video, we're going to begin our lesson on the activation of T lymphocytes or the activation of T cells. We need to recall from some of our previous lesson videos that antigen-presenting cells or APCs such as dendritic cells, for instance, actually have both classes of major histocompatibility complexes or MHCs. This means that these APCs like dendritic cells have both MHC class 1 and MHC class 2. Because they have both classes of MHCs, they can activate either a naive cytotoxic T cell using its MHC class 1, or they could activate a naive helper T cell using its MHC class 2. We also need to recall from some of our previous lesson videos that these activated T cells can then proliferate or multiply to create many identical clones and differentiate into either an effector T cell or a memory T cell. If you do not recall this information from our previous lesson videos, you should definitely go back and check out those older lesson videos before continuing here.

Now that being said, because we know dendritic cells have both MHC class 1 and class 2 and can activate either cytotoxic or helper T cells, we can now look at the two major types of antigens that these dendritic cells could present. In our text down below, this region right here corresponds with the left-hand side of our image down below. This region of our text right here corresponds with the right-hand side of our image down below. Really, there are two major types of antigens that can be presented by dendritic cells. If a dendritic cell presents a harmful antigen on one of its MHCs, either MHC class 1 or MHC class 2, then that dendritic cell will produce what are known as costimulatory molecules on its surface. These costimulatory molecules will only be produced by the dendritic cell if that dendritic cell is presenting a harmful antigen on its MHCs. These costimulatory molecules can be defined as molecules that communicate the danger or the significance of an antigen to the naive T cell. These costimulatory molecules are really just going to enhance the danger sign.

Naive T cells will only become activated by dendritic cells that are presenting harmful antigens and presenting those costimulatory molecules. On the other hand, if a dendritic cell is not presenting a harmful antigen, if the dendritic cell is presenting a harmless antigen on one of its MHCs, then the dendritic cell will not produce these costimulatory molecules. If the costimulatory molecules are not being produced, then the naive T cell is not going to become activated. Instead of becoming activated, the T cell will be known as anergic. The term anergic refers to a cell that becomes unresponsive, and this unresponsive or anergic cell will ultimately undergo apoptosis and get rid of the T cell that would generally target a harmless antigen. What this means is that over time, our bodies will not generate an immune response to harmless antigens, which is exactly what we want. We do not want to generate an immune response towards harmless antigens.

If we take a look at our image down below, we can get a better understanding of the activation of these T lymphocytes. Notice once again, the left-hand side of our image over here corresponds to if a dendritic cell presents a harmful antigen on its MHCs. The right-hand side of our image corresponds to if a dendritic cell presents a harmless antigen on its MHC. If a harmful antigen or an immunogenic antigen is being presented on the MHCs of a dendritic cell, these little blue cells here are our dendritic cells, then, the dendritic cell will also produce these costimulatory molecules on its surface. These costimulatory molecules, you can just imagine, are just these molecules that tell the T cells that this is a dangerous and harmful antigen that needs to be addressed, and an immune response should be generated.

When these helper T cells or cytotoxic T cells detect an antigen presented on a dendritic cell with these costimulatory molecules, then these naive T cells will become activated. The naive T cells will activate, and then they will proliferate and differentiate into either effector T cells or memory T cells and carry out an immune response to get rid of this dangerous or harmful immunogenic antigen. On the right-hand side, if a dendritic cell is presenting a harmless antigen, like for example normal cytoplasmic proteins, then no costimulatory molecules will be produced. Notice that this dendritic cell over here does not have costimulatory molecules. Without those costimulatory molecules, these T cells are not going to become activated. So the naive T cells, instead of becoming activated, they become anergic. Again, the term anergic refers to cells that become unresponsive, and they ultimately undergo apoptosis.

The anergic T cells will undergo apoptosis, and what that means is that we are going to get rid of the populations of T cells that respond to harmless antigens, and that's exactly what we want. We do not want T cells that generate immune responses to harmless, healthy host cells. We only want our immune cells to generate responses towards dangerous immunogenic antigens. This here concludes our brief lesson on the activation of T lymphocytes, and we'll be able to get some more practice applying these concepts and learn even more as we move forward in our course. So I'll see you all in our next video.

Antigen presenting cells, including dendritic cells in lymph nodes, are observed to bind T cells on their surfaces. If the dendritic cell is presenting harmful antigens on its surface and creating co-stimulatory molecules, what will happen to the T cells that bind to the dendritic cell?

• A. The naive T cells that bind the dendritic cell will activate and form effector T cells.
• B. The cytotoxic T cells which bind the dendritic cell will signal the dendritic cell to undergo apoptosis.
• C. The anergic T cells which bind the dendritic cell will undergo apoptosis.
• D. The helper T cells that bind the dendritic cell will deactivate and become naive T cells.

Comic Strip for T-Cell Activation

In this video, we're going to cover this comic strip for T cell activation which we have down below in order to give you guys a fun way to remember when T cells become activated by dendritic cells. And so in this little image or comic strip that we have down below, notice that we have two sides. On the right-hand side over here, this is representing when a dendritic cell presents a harmful antigen to a T cell. And on the right-hand side over here, this is representing when a dendritic cell presents a harmless antigen to a T cell. And so, what you'll notice is that the dendritic cell in this image is this little blue guy over here and over here. This is Mister Dendritic Cell. The helper T cell is going to be up here on the right, and then the antigen being presented is going to be either right here or in this image on right over here. So let's take a look at the one on the left first.

And so what you'll notice is that the left one is presenting a harmful antigen and so we have activation of the T lymphocytes when harmful antigens are presented. And so notice that Mister Dendritic over here, he is hosting this little sweet shop and here comes Virus Vic, a dangerous antigen with a weapon here. And because Virus Vic is dangerous saying "Give me all of your money," notice the dendritic cell is going to produce costimulatory molecules to activate the helper T cell. And so the helper T cell says, "Oh, no. He's clearly harmful and I better activate the reinforcements to generate an immune response and get rid of this dangerous antigen." And so once again, we have activation of T lymphocytes when harmful antigens are presented.

Now, on the right-hand side over here, what we have is no activation of T lymphocytes and this is because a harmless antigen is being presented. And so notice here, we again have Mister Dendritic Cell saying, "Hey, Officer Helper T Cell, look what he did." And you have over here, Stu, which is a self-cell, a harmless cell. And so because a harmless cell or a harmless antigen here is being presented, then the helper T cell is not going to become activated. And so, notice that the helper T cell, Officer T_H here, he's saying, "Oh, that's just my son, and he is harmless." And because he's harmless, there's no need to activate. And because he's not activating, of course, this means that he will become anergic. And anergic means unresponsive, and ultimately, it will end up undergoing apoptosis and that will get rid of the T cells that would respond to harmless antigens. And that helps to ensure that our immune systems are only targeting dangerous and harmful antigens and are not targeting harmless and healthy antigens. And so hopefully this little comic strip here is helpful for you guys to better understand T cell activation and, we'll be able to get some practice applying these concepts as we move forward and learn more as well. So I'll see you all in our next video.

The role of dendritic cells is to:

• A. Activate B cells via antigen presentation.
• B. Present antigens on MHC I only.
• C. Activate the complement system classical pathway.
• D. Perform phagocytosis.
• E. Activate T helper & T cytotoxic cells via MHC I & II.

Before T cells become activated, they require an antigen encounter. Naive T cells are shown to express L-selectin which aids in binding to high endothelial cells expressing L-selectin ligand in regions where they are most likely to encounter an antigen. In which of the following locations are these high endothelial cells most likely to be located?

• A. Bone marrow.
• B. Capillaries.
• C. Lymph nodes.
• D. Heart.

Here’s what students ask on this topic:

What are the roles of MHC class 1 and MHC class 2 in T cell activation?

MHC class 1 and MHC class 2 molecules play crucial roles in T cell activation by presenting antigens to T cells. MHC class 1 molecules present endogenous antigens (from within the cell) to cytotoxic T cells (CD8+), leading to their activation. On the other hand, MHC class 2 molecules present exogenous antigens (from outside the cell) to helper T cells (CD4+), which then activate and coordinate the immune response. The interaction between the T cell receptor (TCR) and the MHC-antigen complex, along with co-stimulatory signals, is essential for the activation, proliferation, and differentiation of T cells into effector and memory cells.

How do co-stimulatory molecules influence T cell activation?

Co-stimulatory molecules are essential for the full activation of T cells. When a dendritic cell presents a harmful antigen on its MHC molecules, it also expresses co-stimulatory molecules on its surface. These molecules provide a second signal that confirms the presence of a dangerous antigen. The interaction between co-stimulatory molecules on the dendritic cell and their receptors on the T cell ensures that the T cell becomes fully activated. Without this co-stimulatory signal, the T cell remains anergic (unresponsive) and eventually undergoes apoptosis, preventing an immune response to harmless antigens.

What happens to T cells when they encounter harmless antigens?

When T cells encounter harmless antigens presented by dendritic cells, no co-stimulatory molecules are produced. As a result, the T cells do not receive the necessary second signal for activation. Instead of becoming activated, these T cells become anergic, meaning they become unresponsive. Anergic T cells eventually undergo apoptosis, which is a programmed cell death process. This mechanism ensures that the immune system does not mount a response against harmless antigens, maintaining immune tolerance and preventing unnecessary or harmful immune reactions.

What is the significance of T cell differentiation into effector and memory cells?

Upon activation, T cells proliferate and differentiate into effector and memory cells. Effector T cells are responsible for immediate immune responses: cytotoxic T cells (CD8+) kill infected cells, while helper T cells (CD4+) coordinate the immune response by activating other immune cells. Memory T cells, on the other hand, provide long-term immunity. They persist in the body after the initial infection has been cleared and can respond more rapidly and effectively if the same antigen is encountered again. This differentiation ensures both an immediate defense and long-term protection against pathogens.

What is the role of dendritic cells in T cell activation?

Dendritic cells are crucial antigen-presenting cells (APCs) that play a key role in T cell activation. They capture and process antigens, then present them on their surface using MHC class 1 or MHC class 2 molecules. When presenting harmful antigens, dendritic cells also express co-stimulatory molecules, which are necessary for the full activation of naive T cells. This interaction leads to the proliferation and differentiation of T cells into effector and memory cells. Dendritic cells thus serve as a bridge between the innate and adaptive immune systems, initiating and regulating the adaptive immune response.