Review of Cytotoxic vs Helper T Cells - Online Tutor, Practice Problems & Exam Prep

Cytotoxic T cells (CD8) and helper T cells (CD4) play distinct roles in the immune response. Cytotoxic T cells target infected nucleated cells by recognizing endogenous antigens presented on MHC class I molecules, inducing apoptosis to eliminate pathogens. In contrast, helper T cells activate antigen-presenting cells (APCs) like macrophages and B cells through exogenous antigens on MHC class II molecules, enhancing the immune response. T lymphocytes remain unresponsive to harmless antigens, ensuring that immune activation occurs only in the presence of threats.

Review of Cytotoxic vs Helper T Cells

In this video, we're going to briefly review the differences between cytotoxic and helper T cells. And we're gonna do that by completing all of the interactive blanks in this table down below. Notice on the far left we have a column that says type of T Cell. Recall there are two main types of T cells. The first are the cytotoxic T cells or TC cells for short, and the second are the helper T cells or TH cells for short. Notice that this entire row right here applies to cytotoxic T cells, and this entire row down below applies to the helper T cells. Recall that in terms of the CD marker, cytotoxic T cells have a CD8 marker on their surface. That's why cytotoxic T cells are sometimes referred to as CD8 cells. Whereas helper T cells have a CD4 marker on their surface, and they're referred to as CD4 cells occasionally.

In terms of the antigen-presenting proteins, recall that cytotoxic T cells are capable of recognizing and responding to antigens presented on MHC class I molecules. On the other hand, helper T cells are capable of recognizing and responding to antigens presented on MHC class II molecules. Recall that the MHC class I molecules are going to be found on all nucleated cells, and this is partially what allows cytotoxic T cells to target all of our nucleated cells. On the other hand, MHC class II molecules are not found on all nucleated cells. Instead, they're found on some of our cells, specifically the antigen-presenting cells, such as dendritic cells, macrophages, and B cells.

In terms of the antigen source, notice that MHC class I molecules are going to be presenting endogenous antigens, or in other words, antigens that are inside the target cell. So for example, antigens from a virus that is infecting the inside of a target cell. Now, MHC class II molecules, on the other hand, present exogenous antigens or antigens that are going to be outside of the target cell initially. And so you can see here these antigens originated from outside of the target cell. Now they can be internalized and processed and then presented on these MHC class II molecules. But again, these antigens would have originated and be causing harm on the outside of cells. Notice that in terms of the target cells, that once again because MHC class I molecules are on all nucleated cells, it allows cytotoxic T cells to pretty much target any of our nucleated cells, which is practically all of our cells. Now on the other hand, MHC class II molecules are only found on those APCs, antigen-presenting cells. And it turns out that the helper T cells, they mainly are going to be targeting the APCs that include B cells and macrophages.

In terms of the functions, recall that cytotoxic T cells, their function is to induce target cell apoptosis. Inducing apoptosis of infected host cells, causing a cell that is infected with an intracellular pathogen like a virus to undergo apoptosis in order to eliminate the infected cell but also eliminate the pathogen, the intracellular pathogen. Now helper T cells, on the other hand, they do not induce apoptosis. Instead, they're all about activating or stimulating a target cell to improve that target cell's immune capabilities. And so they can target B cells and target macrophages to help activate those B cells and macrophages, and again either lead to an immune response or lead to an improved immune response. This here concludes our brief review of cytotoxic versus helper T cells. And again, we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.

Comic Strip for Effects of T Cell Functions

In this video, we're going to briefly cover our comic strip for T cell functions. Notice that down below we have this portion of our image that corresponds to the same comic strip that we covered in our previous lesson video. Here in this image, we've only added this little section comic, recall that T lymphocytes will not be activated if harmless antigens are being presented. Notice that our dendritic cell over here is presenting an antigen that is harmless, it's little stew, which is a self cell, a harmless antigen. Notice that the T cell here recognizes that this is a harmless antigen, that there's no need to activate. And so it remains in an anergic or unresponsive state. It will not generate an immune response towards harmless antigens.

On the other hand, if a harmful antigen is being presented, such as virus VIC right here, then the dendritic cell is going to produce costimulatory molecules that will activate the T lymphocytes. Notice he's saying here he is harmful and because this is a harmful antigen it is going to be generating an immune response. It activates reinforcements, and ultimately, in our new part of the image, the activated T lymphocytes are going to be able to create an effective immune response to ultimately eliminate the threat. By eliminating the threat here what you can see is that these other immune components have been assembled here. We have the CD8 cells, we have the CD4 cells, and the CD4 cells can activate macrophages and B cells and things of those natures to generate an effective immune response to eliminate the pathogen and destroy the pathogen so that it no longer is effective and cannot cause us harm.

As we'll talk about moving forward in our course, we're going to learn how sometimes our immune system can cause unintentional harm to our own self. That is not a good thing about our immune system. Yes, it's very effective at eliminating pathogens, but occasionally it can cause our own tissues harm. We'll be able to talk about that aspect of our immune system more as we move forward in our course. But for now what you can see here is that again T lymphocytes are not activated with harmless antigens; they're activated with harmful antigens, and once those T lymphocytes are activated they will work to eliminate the threat. We'll be able to get some practice applying these concepts and learn more as we move forward in our course. So I'll see you all in our next video.

Helper T cells respond to antigens from _____ and cytotoxic T cells respond to antigens from _____.

• A. Macrophages; B cells.
• B. Class II MHC; class I MHC.
• C. Viruses; bacteria.
• D. Class I MHC; class II MHC.

Which of the following cells is capable of specifically responding to a nearly infinite number of epitopes?

• A. B and T cells.
• B. Plasma cells.
• C. T cytotoxic cells.
• D. All of the above.

Here’s what students ask on this topic:

What are the main differences between cytotoxic T cells and helper T cells?

Cytotoxic T cells (CD8) and helper T cells (CD4) have distinct roles in the immune response. Cytotoxic T cells recognize endogenous antigens presented on MHC class I molecules found on all nucleated cells. Their primary function is to induce apoptosis in infected cells, eliminating intracellular pathogens like viruses. In contrast, helper T cells recognize exogenous antigens presented on MHC class II molecules, which are found on antigen-presenting cells (APCs) such as macrophages and B cells. Helper T cells activate these APCs, enhancing their immune capabilities and promoting a more effective immune response.

How do cytotoxic T cells recognize infected cells?

Cytotoxic T cells (CD8) recognize infected cells by detecting endogenous antigens presented on MHC class I molecules. These MHC class I molecules are found on the surface of all nucleated cells. When a cell is infected by an intracellular pathogen, such as a virus, it processes and presents fragments of the pathogen (antigens) on its MHC class I molecules. Cytotoxic T cells bind to these antigen-MHC class I complexes through their T cell receptors (TCRs) and CD8 co-receptors, allowing them to identify and target the infected cells for apoptosis.

What role do helper T cells play in the immune response?

Helper T cells (CD4) play a crucial role in the immune response by activating and enhancing the functions of other immune cells. They recognize exogenous antigens presented on MHC class II molecules, which are found on antigen-presenting cells (APCs) like macrophages and B cells. Upon activation, helper T cells secrete cytokines that stimulate these APCs, improving their ability to phagocytose pathogens, present antigens, and produce antibodies. This activation leads to a more robust and effective immune response, helping to eliminate extracellular pathogens and coordinate the overall immune defense.

Why do T lymphocytes remain unresponsive to harmless antigens?

T lymphocytes remain unresponsive to harmless antigens to prevent unnecessary immune responses that could lead to autoimmunity or tissue damage. When a T cell encounters a harmless antigen, such as a self-antigen, it does not receive the necessary co-stimulatory signals from antigen-presenting cells (APCs). This lack of co-stimulation keeps the T cell in an anergic or unresponsive state. This mechanism ensures that T cells are only activated in the presence of harmful antigens, such as those from pathogens, thereby maintaining immune tolerance and preventing damage to the body's own tissues.

What is the significance of MHC class I and MHC class II molecules in T cell activation?

MHC class I and MHC class II molecules are crucial for T cell activation as they present antigens to T cells. MHC class I molecules present endogenous antigens to cytotoxic T cells (CD8), which are found on all nucleated cells. This allows cytotoxic T cells to recognize and eliminate infected cells. MHC class II molecules present exogenous antigens to helper T cells (CD4), which are found on antigen-presenting cells (APCs) like macrophages and B cells. This interaction activates helper T cells, which then enhance the immune response by stimulating APCs and other immune cells. The specificity of these interactions ensures that the appropriate T cell response is generated based on the type of antigen presented.