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

The activation of T lymphocytes involves antigen-presenting cells (APCs) like dendritic cells, which express both MHC class 1 and class 2. When presenting harmful antigens, these cells produce co-stimulatory molecules, activating naive T cells to proliferate and differentiate into effector or memory T cells. Conversely, presenting harmless antigens leads to T cell anergy, resulting in unresponsive cells that undergo apoptosis. This mechanism ensures the immune system targets only dangerous antigens, maintaining tolerance to harmless substances.

Activation of T Lymphocytes

In this video, we're going to begin our lesson on the activation of T lymphocytes or the activation of T cells. And we need to recall from some of our previous lesson videos that antigen presenting cells or APCs such as dendritic cells for instance, they actually have both classes of major histocompatibility complexes or MHCs. Which means that these APCs like dendritic cells, they have both MHC class 1 and MHC class 2. And because they have both classes of MHCs, they can activate either a naive cytotoxic T cell using its MHC class 1, or it could activate a naive helper T cell using its MHC class 2. Now we also need to recall from some of our previous lesson videos that these activated T cells, they can then proliferate or multiply to create many identical clones and differentiate into either an effector t cell or a memory t cell. And so 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 you continue 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 cell, we can now look at the 2 major types of antigens that these dendritic cells could present. And so notice that in our text down below, this region right here corresponds with the left-hand side of our text of our image down below. And this region of our text right here corresponds with the right-hand side of our image down below. And so, really, there are 2 major types of antigens that can be presented by dendritic cells.

Now 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. And so these costimulatory molecules will only be produced by the dendritic cell if that dendritic cell is presenting a harmful antigen on its MHCs. Now these costimulatory molecules can be defined as molecules that communicate the danger or the significance of an antigen to the naive t cell. And so these costimulatory molecules are really just going to enhance, the danger sign. Naive t cells, they 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. And 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. And so the term anergic is referring 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. And 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. And so if we take a look at our image down below, we can get a better understanding of the activation of these T lymphocytes. And so 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. And the right-hand side of our image corresponds to if a dendritic cell presents a harmless antigen on its MHC. And so if a harmful antigen or an immunogenic antigen is being presented on the MHCs of a dendritic cell, so these little blue cells here are our dendritic cells, then, the dendritic cell will also produce these costimulatory molecules on its surface. And the costimulatory molecules, you can just imagine, are just these molecules that tell the t cells that this is dangerous and harmful antigen that needs to be addressed, and an immune response should be generated. And so when these helper T cells or cytotoxic T cells when they 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. Now 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. And so notice that this dendritic cell over here does not have costimulatory molecules. And 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. And again, the term anergic refers to cells that become unresponsive, and they ultimately undergo apoptosis. And so the allergic 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. And so 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. And 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 to the right, and then the antigen being presented is going to be either right here or in this image on the 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 co-stimulatory 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 doing 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. In contrast, MHC class 2 molecules present exogenous antigens (from outside the cell) to helper T cells (CD4+), which then activate and proliferate. This differentiation ensures that the immune response is appropriately directed, with cytotoxic T cells targeting infected or malignant cells and helper T cells coordinating the broader immune response.

How do co-stimulatory molecules influence T cell activation?

Co-stimulatory molecules are essential for the full activation of naive 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, ensuring that the T cell becomes fully activated. Without this co-stimulatory signal, the T cell remains anergic (unresponsive) and eventually undergoes apoptosis. This mechanism prevents the immune system from reacting to harmless antigens, maintaining tolerance and preventing autoimmunity.

What happens to T cells when they encounter harmless antigens?

When T cells encounter harmless antigens presented by dendritic cells, they do not receive the necessary co-stimulatory signals. As a result, these T cells become anergic, meaning they are unresponsive. Anergic T cells eventually undergo apoptosis, a process of programmed cell death. This mechanism ensures that the immune system does not mount a response against harmless antigens, such as self-proteins or benign environmental substances, thereby preventing unnecessary inflammation and autoimmunity.

What is the significance of T cell anergy in immune tolerance?

T cell anergy is crucial for maintaining immune tolerance, which is the immune system's ability to avoid attacking the body's own tissues and harmless substances. When T cells encounter harmless antigens without co-stimulatory signals, they become anergic and eventually undergo apoptosis. This process eliminates T cells that could potentially target self-antigens or benign environmental antigens, thereby preventing autoimmune diseases and ensuring that the immune response is directed only against harmful pathogens.

How do dendritic cells contribute to the activation of T lymphocytes?

Dendritic cells are key antigen-presenting cells that play a pivotal role in the activation of T lymphocytes. They capture and process antigens, presenting them on 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 dual signaling ensures that T cells are activated only in the presence of a genuine threat, leading to their proliferation and differentiation into effector and memory T cells, which are essential for an effective immune response.