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 recognize endogenous antigens presented on MHC class I molecules, targeting all nucleated cells to induce apoptosis in infected cells. In contrast, helper T cells respond to exogenous antigens on MHC class II molecules found on antigen-presenting cells, activating B cells and macrophages to enhance immune capabilities. T lymphocytes remain unresponsive to harmless antigens, ensuring immune responses are directed only against harmful 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 going to do that by completing all of the interactive blanks in this table down below. And so notice on the far left we have a column that says type of T Cell. Recall there are 2 main types of T cells. The first are the cytotoxic T cells or T_c cells for short, and the second are the helper T cells or T_h cells for short. Now notice that this entire row right here applies to cytotoxic T cells, and this entire row down below applies to the helper T cells. Now recall that in terms of the CD marker, cytotoxic T cells have a CD8 marker on their surface. And so 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.

Now 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. And, on the other hand, helper T cells are capable of recognizing and responding to antigens presented on MHC class II molecules. Now recall that the MHC class I molecules are going to be found on all nucleated cells, And this is partially what allows for cytotoxic T cells to target all of our nucleated cells. Now 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.

Now 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's infecting the inside of a target cell. Now MHC class II molecules on the other hand, they 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, 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.

And, 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. And, 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. And so 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 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 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. Now, 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 co-stimulatory molecules that will activate the T lymphocytes. And notice he's saying here he is harmful and because this is a harmful antigen it is going to be generating an immune response. So 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 CD 8 cells, we have the CD 4 cells, and the CD 4 cells can activate macrophages and B cells and things of those nature 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. Now as we'll talk about more moving forward in our course, we're going to learn how sometimes our immune system can cause unintentional harm to our own self. And so 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. 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. And so 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, which are found on all nucleated cells. Their primary function is to induce apoptosis in infected cells, thereby eliminating intracellular pathogens. In contrast, helper T cells respond to exogenous antigens presented on MHC class II molecules, which are found on antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells. Helper T cells activate these APCs to enhance their immune capabilities, such as stimulating B cells to produce antibodies and activating macrophages to destroy pathogens.

How do cytotoxic T cells recognize infected cells?

Cytotoxic T cells (CD8) recognize infected cells through the presentation of endogenous antigens on MHC class I molecules. These MHC class I molecules are present on all nucleated cells and display peptide fragments from proteins synthesized within the cell. When a cell is infected by a virus or other intracellular pathogen, these peptides include fragments of the pathogen. The cytotoxic T cell's T-cell receptor (TCR) binds to the antigen-MHC class I complex, allowing the T cell to recognize and target the infected cell for apoptosis, thereby eliminating the pathogen.

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 found on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. Upon activation, helper T cells secrete cytokines that stimulate these APCs. For example, they help B cells to produce antibodies and activate macrophages to destroy pathogens. This coordination ensures a robust and effective immune response against extracellular pathogens.

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 dendritic cell presents a harmless antigen, it does not produce co-stimulatory molecules required for T cell activation. As a result, the T cell remains in an anergic or unresponsive state. This mechanism ensures that the immune system targets only harmful threats, such as pathogens, while ignoring self-antigens and other non-threatening substances.

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

MHC class I and MHC class II molecules are crucial for T cell function as they present antigens to T cells, guiding the immune response. MHC class I molecules present endogenous antigens to cytotoxic T cells (CD8), which then target and induce apoptosis in infected cells. These molecules are found on all nucleated cells. MHC class II molecules present exogenous antigens to helper T cells (CD4), which activate and enhance the functions of antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells. This activation is essential for a coordinated and effective immune response against extracellular pathogens.