Functions of T Lymphocytes: Study with Video Lessons, Practice Problems & Examples

Cytotoxic T cells (TC cells) target infected host cells by recognizing immunogenic antigens presented on MHC class I molecules, inducing apoptosis to eliminate pathogens. Helper T cells (TH cells) activate other immune cells, enhancing responses against pathogens. TH1 cells activate macrophages and cytotoxic T cells, TH2 cells target multicellular pathogens like helminths, and TH17 cells respond to extracellular bacteria. This coordinated immune response is crucial for effective pathogen elimination and maintaining homeostasis within the immune system.

Functions of Cytotoxic T Cells

In this video, we're going to talk more details about the functions of T lymphocytes. And in this video, we're going to first start off by talking about the functions of cytotoxic T cells or TC cells, and then later in our lesson in a different video, we'll talk about the functions of the helper T cells. Now first, we need to recall from some of our previous lesson videos that cytotoxic T cells or TC cells are important for targeting infected host cells that are presenting intracellular pathogens or antigens from intracellular pathogens on MHC class ones. And these cytotoxic T cells will induce apoptosis in these infected host cells, eliminating the infected host cell and ultimately eliminating the intracellular pathogen within them as well. Now, these cytotoxic T cells have the ability to distinguish between uninfected healthy host cells and infected dangerous host cells. And they can do that by what is being presented on the MHC class 1 molecules. On uninfected cells or healthy host cells, these are going to present non-immunogenic self-made peptides. And so what that means is that these are going to be presenting normal proteins that belong to us on their MHC class 1s. And self-made peptides are non-immunogenic which means that they do not elicit an immune response. And so again, we know from our previous lesson videos that these T cells would become anergic or unresponsive if they did target uninfected cells. Now, infected cells that are infected with some kind of intracellular pathogen, like for example a virus, these are going to be presenting immunogenic antigens, immunogenic microbial antigens. And immunogenic microbial antigens, as their name implies, they do elicit an immune response or they do cause an immune response, and those are going to be the ones that the T cells are going to be focusing their immune response on, the infected cells. And so, when a cytotoxic T cell binds an infected cell presenting an immunogenic microbial antigen, that cytotoxic T cell is going to release a series of proteins and molecules, like for example, proteases, which are proteins that degrade other proteins, and perforin, which is referring to molecules that are capable of creating pores in the infected cell. And these pores in the infected cell allow proteases to enter into the infected cell via those pores. And, again, these proteases are proteins themselves that degrade cellular proteins within the infected cell. And by degrading all of these cellular proteins within the infected cell, that ultimately will induce apoptosis in the infected cell. And so by the cytotoxic T cell inducing apoptosis in infected cells, apoptosis is a way to control the killing of infected cells without exposing the pathogens to nearby healthy cells. And so, these cytotoxic T cells, they can also release cytokines or communicating molecules to neighboring cells, activating macrophages and increasing antigen presentation on dendritic cells, ultimately helping to boost the immune response even more. And so, if we take a look at our image down below, we can get a better understanding of the functions of these cytotoxic T cells. And notice that our image is broken up into 2 sections. We have the top section here in blue and then we have this bottom section down below here in pink. And what you'll notice is that in the top section what we have is an uninfected cell. This is a healthy cell and it is going to be presenting normal cytoplasmic proteins on its MHC class 1. And so because this uninfected cell is presenting normal proteins, this cytotoxic T cell over here is or TC cell or CD8 cell, in other words, is not going to generate an immune response towards these uninfected healthy cells and so no immune response is generated here. However, if the cytotoxic T cell over here recognizes, these microbial antigens, these dangerous microbial antigens, like for example, antigens from a virus, then it will be able to, determine that this is an infected host cell. And again, that is because this infected host cell is presenting immunogenic microbial antigens or microbial antigens that are capable of generating an immune response. And so the cytotoxic T cell again will release the proteases, will release the perforin, and, notice that it will allow these molecules, which sometimes are referred to as death packages, to allow those molecules to enter into the infected cell and cause apoptosis of the infected cell. And again, by causing apoptosis of the infected cell it gets rid of the infected cell, but it also helps to get rid of and eliminate the pathogen that is inside of it. And so it is a way to help defend ourselves from the pathogens. And so this here concludes our brief introduction to the functions of cytotoxic T cells and their ability to cause apoptosis in infected cells. And we'll be able to get some practice applying these concepts, and then talk about the functions of helper T cells as we move forward in our course. And so I'll see you all in our next video.

Which statement is FALSE about cytotoxic T cells?

• A. They stimulate B cells.
• B. They destroy virus-infected cells.
• C. They recognize MHC I bound antigens on APCs.
• D. They induce apoptosis in infected cells.

$T_c$ cells recognize epitopes only when the latter are held by

• A. MHC proteins.
• B. B cells.
• C. Interleukin-2.
• D. Granzyme.

Types of Helper T Cells

In this video, we're going to briefly introduce some of the different types of helper T cells or TH cells. And so depending on the signals provided by a dendritic cell, the naive helper T cells can differentiate into one of many different effector helper T cell subtypes. And so we're not going to cover all of the possible helper T cell subtypes, but down below, notice we have a table with 3 of the different subtypes of helper T cells. And so each subtype of effector helper T cell can produce different cytokines or communicative signals that can control an immune response in slightly different ways. And so if we take a look at our image down below of this table of the types of helper T cells, notice that we're showing you 3 different types, TH1, TH2, and TH17 cells. And, again, there are many, many types that are not being listed here. And so we're only going to briefly cover these 3 to give you a sense of what this could look like. And so for these TH1 cells, these are cells that respond to intracellular pathogens and so, for example, viruses, that are infecting a cell and they can do so by activating macrophages and also by activating cytotoxic T cells or TC cells and we know that cytotoxic T cells target intracellular pathogens. And so, sometimes these helper T cells can actually help to activate other T cells, other cytotoxic T cells. And then, this is an example of, a class of helper T cell that's capable of doing that. Now the TH2 cell, as you can see here, these can actually respond to multicellular pathogens. For example, helminths, these, worms and, they're multicellular and they can do so by recruiting and activating eosinophils and basophils which again can target these multicellular pathogens like helminths. And then notice that the TH17 helper T cells, those can respond to extracellular pathogens or pathogens on the outside of cells, like for example, bacteria that are on the outside of cells by recruiting and activating neutrophils and again these neutrophils can, go on to, target these bacteria, these extracellular pathogens and eliminate them. And so again, there are a variety of different helper T cells all with slightly varied functions. However, it is important to note that moving forward in our lesson, we are going to consider the functions of all helper T cells subtypes as a group collectively. So we're not going to focus in on any individual one of these, subtypes of helper T cells. We'll be focusing on them collectively as a group. And so that being said, this here concludes our brief introduction to some of the different types of helper T cells, and we'll be able to get some practice and learn more as we continue to move forward in our course. So I'll see you all in our next video.

Functions of Helper T Cells: Activation of Macrophages

In this video, we're going to talk more generally about the functions of helper T cells, or T_H cells, and we're going to focus specifically on the activation of macrophages. Now first, we need to recall from some of our previous lesson videos that naive helper T cells, or inactive helper T cells, can become activated by dendritic cells that are antigen-presenting and presenting antigens on their MHC class 2 molecules. Now, unlike the cytotoxic T cells whose function is to mainly induce apoptosis in infected cells, helper T cells don't induce apoptosis in other cells. Instead, helper T cells or T_H cells, they are going to make cytokines or communication signals that ultimately help to stimulate and activate other immune cells, improving the immune capabilities of immune cells around it. And so it can improve or activate the immune cells such as macrophages, naive cytotoxic T cells, and B cells as well.

Now here in this video, we're going to specifically focus on the ability for helper T cells to activate macrophages. And so macrophages, they are routinely engulfing, degrading, and processing invading pathogens in many of our different tissues. And this is something that they do on a routine basis. And what they can do is they can also present these pathogens as antigens on their MHC class 2 molecules. Now effector helper T cells are capable of binding to the antigens presented on MHC class 2, and that will activate the helper T cells. And the activated helper T cells, again, will produce cytokines that are capable of activating other immune cells or stimulating the macrophages even further, improving their immune capabilities.

And so these stimulated or activated macrophages have increased production of lysozymes, which we'll call organelles involved with destruction, degrading, and also increased production of antimicrobials that can destroy invaders more effectively. And so the helper T cell helps the macrophage destroy invaders more effectively. Now the helper T cells can also release additional cytokines to get rid of invaders even better. And so if we take a look at our image down below, we can get a better understanding of the functions of these helper T cells. Now notice on the left over here what we're showing you is a macrophage. And so really the top half of this image here is just a cartoon version of another they're showing the same thing just in a different way. And so what we have is a macrophage, and the macrophage is not yet activated. And so these macrophages, they are commonly and routinely engulfing, degrading, and processing pathogens. So they have an invading pathogen here and the macrophage will engulf it, degrade it, break it down, and then present it on its surface, at on these MHC class 2 molecules. And so you can see here our macrophage is presenting the antigens on its MHC class 2 molecules.

And so, what you'll notice is that when a helper T cell recognizes the antigen on an MHC class 2 molecule, the helper T cell can release cytokines. And these cytokines here can have an impact on the macrophage, and it can activate the macrophage. And so notice here we have our helper T cell is somewhat presenting these cytokines to the macrophage, and the macrophage is taking it in. And that improves the destructive capabilities of the macrophage so that we have an activated or stimulated macrophage. And so now it's capable of killing the invaders even more effectively. And so down below what we're showing you is again the activated or the stimulated macrophage with improved, destructive immune abilities. And so really that's what these helper T cells do; they help other immune cells improve their destructive immune abilities even further.

And so this concludes our brief lesson on the general functions of helper T cells and their ability to activate macrophages. And we'll be able to get some practice applying these concepts as we move forward in our course. So, I'll see you all in our next video.

Why are cytokines important signals for cell-mediated immunity?

• A. Cell-mediated immunity requires immune cells to communicate to perform most efficiently.
• B. Cytokines are signals that allow immune cells to communicate.
• C. Effector cells, like CD4 cells, use cytokines to activate other immune cells.
• D. Cytokines signal to specific immune cells to increase their destructive properties and destroy pathogens.
• E. A and B.
• F. C and D.
• G. All of the above.

T______ cells assist in the functions of certain B cells and other T cells.

• A. sensitized.
• B. cytotoxic.
• C. helper.
• D. natural killer.

Which type of helper T cells are involved in fighting extracellular pathogens?

• A. $T_H1$
• B. $T_H2$
• C. $T_H17$
• D. A and B.
• E. B and C.
• F. All types of helper T cells help fight extracellular pathogens.

Here’s what students ask on this topic:

What are the primary functions of cytotoxic T cells (TC cells)?

Cytotoxic T cells (TC cells) primarily target and eliminate infected host cells. They recognize immunogenic antigens presented on MHC class I molecules of infected cells. Upon recognition, TC cells release proteins such as perforin and proteases. Perforin creates pores in the infected cell membrane, allowing proteases to enter and degrade cellular proteins, ultimately inducing apoptosis. This controlled cell death eliminates the infected cell and the intracellular pathogen within it. Additionally, TC cells release cytokines to activate neighboring immune cells, such as macrophages and dendritic cells, enhancing the overall immune response.

How do helper T cells (TH cells) activate macrophages?

Helper T cells (TH cells) activate macrophages by recognizing antigens presented on MHC class II molecules of macrophages. Upon binding, TH cells release cytokines that stimulate macrophages, enhancing their immune capabilities. Activated macrophages increase the production of lysosomes and antimicrobials, improving their ability to degrade and destroy pathogens. This activation process allows macrophages to more effectively eliminate invaders, contributing to a stronger immune response. The interaction between TH cells and macrophages is crucial for maintaining an effective defense against pathogens.

What are the different subtypes of helper T cells and their functions?

Helper T cells (TH cells) differentiate into various subtypes based on signals from dendritic cells. Three main subtypes include TH1, TH2, and TH17 cells. TH1 cells respond to intracellular pathogens, such as viruses, by activating macrophages and cytotoxic T cells. TH2 cells target multicellular pathogens, like helminths, by recruiting and activating eosinophils and basophils. TH17 cells respond to extracellular bacteria by recruiting and activating neutrophils. Each subtype produces specific cytokines that tailor the immune response to effectively combat different types of pathogens.

How do cytotoxic T cells distinguish between infected and uninfected cells?

Cytotoxic T cells distinguish between infected and uninfected cells by recognizing the antigens presented on MHC class I molecules. Uninfected cells present non-immunogenic self-made peptides, which do not elicit an immune response. In contrast, infected cells present immunogenic microbial antigens from intracellular pathogens. These microbial antigens trigger an immune response from cytotoxic T cells, leading to the release of perforin and proteases that induce apoptosis in the infected cells. This selective recognition ensures that only infected cells are targeted and eliminated.

What role do cytokines play in the function of helper T cells?

Cytokines are crucial signaling molecules produced by helper T cells (TH cells) to regulate and enhance the immune response. When TH cells recognize antigens on MHC class II molecules, they release cytokines that activate other immune cells, such as macrophages, cytotoxic T cells, and B cells. These cytokines improve the immune capabilities of these cells, enabling them to more effectively destroy pathogens. For example, cytokines can increase the production of lysosomes and antimicrobials in macrophages, enhancing their pathogen-killing abilities. Overall, cytokines coordinate and amplify the immune response, ensuring efficient pathogen elimination.