in this video we're going to begin our introduction to T lymphocytes or t cells. And so first we need to recall from some of our previous lesson videos that t lymphocytes or T cells play an important role in cell mediated immunity which recall is a part of adaptive immunity. And so cell mediated immunity is going to be targeting and destroying intracellular pathogens or disease causing agents that have made their way inside of a host cell. And it does this by using once again T cells or t lymphocytes. And so down below what we're going to do is focus on this image. But we're going to continue to introduce and talk about T cells and T lymphocytes as we move forward. And so what you'll notice here is we're showing you the primary lymphoid organs here and recall that T cells are going to be developed in the thymus And after these t cells are fully developed the naive forms of the T cells or the inactive forms of the T cells will migrate to the secondary lymphoid organs like tonsils or lymph nodes or the spleen. And so really what we're going to learn moving forward is that there are two major types of T cells. There are side a toxic T cells or T. C cells and then there are also helper T cells or T. H cells. And as we move forward in our course we will differentiate between these two types of T cells. Now here we're showing you that both the side a toxic T cell and the helper T cell need to be activated by an antigen presenting cell such as a dendritic cell. And so here we're showing you a dendritic cell presenting antigens on its surface so that they can activate these T cells. And upon activation of the T cells these T cells will begin to divide and differentiate into either memory T cells. So notice that there are both memory side A toxic T cells memory TC cells but there are also memory T. H cells as well. Or memory helper T cells. And in addition to generating these helper these memory cells they also will differentiate into affect er cells as well affect their helper T cells and also affect their side a toxic T cells. And so what you'll notice is that it's these defector t cells that can go on to carry out the immune functions and the memory t cells are just stored for uh generating an immune response to a secondary infection. A potential secondary infection. Now notice that the B cells are being shown over here on the right hand side. But we're going to talk about the B cells later in our course after we finished talking about the T cells. And so everything that we've discussed here in this video about the T cells we're going to break down and explain in more details as we move forward. And so this year concludes our brief introduction to T lymphocytes. And again we'll continue learning about them as we go forward. So I'll see you all in our next video.

in this video we're going to talk more details about T cell receptors. And so first we need to recall from some of our previous lesson videos that T cells, our immune system cells that play a huge role in adaptive cell mediated immunity. And recall that these T cells they develop in the thymus. And these T cells have thousands of identical T cell receptors that are embedded in their membranes. Now these T cell receptors are commonly abbreviated as a T C. R. S. And so these T cell receptors or T C. R. S. R. Again, receptors that allow T cells to recognize and attack very specific intracellular pathogens or pathogens that are on the inside of a host cell. Now also recall from some of our previous lesson videos that these T cell receptors are not capable of binding free antigens. These T cell receptors can only bind to presented antigens or antigens that are presented on the surface of antigen presenting cells. Now in terms of the structure of these T crs, each T C. R consists of two polyp peptide chains and alpha chain and a beta chain. And these two polyp peptide chains are linked via special types of covalin bonds known as di sulfide bonds. And also each of these polyp peptide chains contains two different regions. The first region that each of them contains is a variable region and the second region that each of them contains is a constant region and we'll be able to see all of this down below in our image in a moment. Now the variable region of each of the polyp peptide chains as its name implies is a region of the polyp peptide chain that varies between different T. C. R. S. And so different T C. R. S. Will have different variable regions. And also this variable region is the region that will directly bind to the presented antigen. Once again, the T C. R. S. Are only capable of binding to presented antigens. They are not capable of binding to free antigens. Now the amino acid sequence variation that's in this variable region accounts for the many different types of antigens that different T crs combined and so different T cRS are capable of binding to different antigens. But once again a single T cell will have thousands of identical T cell receptors which means that a single T cell is only capable of generating an immune response towards one very specific antigen. Now the constant region is the second region that each of these polyp peptide chains contains. And the constant region as its name implies, is going to be a region that remains relatively constant uh in terms of its amino acid sequence. And also this constant region is the region that's closest to the cell surface and once again and once again it is going to be very constant in its amino acid sequence. And so if we take a look at our image down below, we can get a better understanding of these T cell receptors. And so notice on the left hand side over here we're showing you a T cell and again recall that these t cells are going to have thousands of identical t cell receptors. And notice that here we're showing you these t cell receptors that these t cells can have. And notice that this if we zoom into this particular t cell receptor we can see that is what this image is showing a zoom in of this particular t cell receptor. And when we look at the entire t cell receptor you'll notice that it has those two polyp peptide chains. It has this alpha chain over here on the left and then it has this beta chain over here on the right and the two chains are linked via di sulfide bonds. Now notice that each of these chains contains a variable region which is here at the top and this variable region that you see right here again is going to vary from one T. C. R. To another T cr um vary between different t crs and also it's going to allow for the binding to a presented antigen. Um Now down below in this green region what we have is the constant region of these polyp peptide chains and the constant region is going to remain relatively constant from one T. C. R. To a different T. C. R. And so this here concludes our brief introduction to these t cell receptors. And once again we'll be able to get some practice applying these concepts and learn more about t lymphocytes as we move forward in our course. And so I'll see you all in our next video.

in this video we're going to introduce naive T cells affect er T cells and memory T cells and will also talk about how these naive T cells can actually become a defector and memory T cells. Now it is important to note that before a developed T. Cell encounters it's presented antigen that it's supposed to generate an immune response to. That T cell exists in an inactive form that we referred to as a naive T cell. And so a naive T. Cell is a T cell that is inactive because it has not yet encountered a presented antigen. And so before the T cell can actually generate an immune response the naive T cell must become activated. And so when naive T cells encounter presented antigen on an antigen presenting cells such as for example a dendritic cell then that naive T cell can become activated. And so activated T cells are capable of doing two things they're capable of proliferating or multiplying to create more of identical clones of itself. And they're also capable of differentiating or changing their phenotype to become a slightly different cell types. And so when they differentiate to change their phenotype they can differentiate into either an effect er T. Cell or they could differentiate into a memory T. Cell. And so the effect er T cells are going to be short lived cells and these uh infected t cells that are short lived they generate an immediate immune response to the first infection upon encountering the antigen for the first time. Now the memory T cells on the other hand these cells are long lived cells that can survive for many many years perhaps even the lifespan of the human being. Now these memory T cells that are long lived. They also remember the antigen and by remember the antigen. What that means is that they do not respond to the first infection. However they will generate a very fast response to any future infection such as a secondary infection and so that helps to create a stronger immune response when you are exposed to one of these antigens a second time. And so we'll get to talk more about these details here as we move forward in our course. Now it is also important to note that there are two main types of defector T cells and two main types of memory T cells. And those are going to be the site a toxic and helper T cells. And so that means that there are defector side a toxic effect er T cells, cida toxic memory T cells and there are also helper affect their T cells and helper memory T cells. And so if we take a look at our image down below we can get a better understanding of all of these T cells. And so notice on the left hand side over here we're focusing in on the side a toxic T cells and remember that the side a toxic T cells have an effect er side a toxic T cell and there's also memory side a toxic T cells And so notice that these T cells originally they start off in their naive forms. And so here we have a naive T cell and recall that the naive T cell is going to be inactive because it has not yet encountered. It's presented antigen. However when the naive T. Cell is uh introduced to its presented antigen by an antigen presenting cells such as dendritic cell. And so this cell over here is our antigen presenting cell presenting an antigen to this naive T cell. When the naive T. Cell recognizes a presented antigen then that can lead to T cell activation which is what we see right here. And so this T cell activation will allow the the T. Cell to proliferate divide and differentiate into either an effect. Er side a toxic T cell. So this would be a sido toxic ah an effective side a toxic T cell being represented right here and notice that we're showing this side a toxic T cell somewhat like the grim reaper here because it's going to have some immune responses that are kind of similar to a grim reaper. And we'll be able to talk about that later as we move forward in our course And so notice that the site a toxic effect er T cell is capable of generating an immune response. And later we'll also get to talk about how these side a toxic T cells are also referred to as Cd eight cells which is why this character here has a C. D. A. On it now again in addition to differentiating into effect er side a toxic T cells. The Cida toxic the activation of the naive side. A toxic T. Cell could also lead to the differentiation into a memory side a toxic T. Cell which is what we have over here. And the memory side of toxic T cell does not respond to the very first infection. Instead it's a long live cell that is capable of responding faster to future infections. And so notice that this memory side of toxic T cell here is saying I will remember this antigen if we encounter it again in the future. Now over here on the right hand side we have an analogous image but instead of Cida toxic T cells these are for helper T cells. And so again the helper T cells also have an effect er helper T cells and a memory helper T cells. And so again it originally starts off as a naive T cell but upon recognizing and being presented its antigen by an antigen presenting cell like a dendritic cell. The naive T cell which is inactive can become activated So we have T cell activation and again that leads to the differentiation and the proliferation of these cells So it can differentiate into a helper T cell which is the effect er T cell and or it could differentiate into a memory T cell which again is going to generate a faster response and future infections. Now the helper T cell is going to uh as its name implies help activate other components of the immune system and so we'll be able to talk more about the actual um uh immune responses of these T cells as we move forward in our course. But for now this here has concluded our lesson on naive T cells and how these naive T cells can become activated to differentiate into either effect er T cells or memory T cells and how there are also side a toxic and helper versions of each of these T cells. And so this year concludes this lesson and we'll be able to get some practice applying this as we move forward. So I'll see you all in our next video.

in this video we're going to further differentiate between Cida toxic T cells and helper T cells. And so we need to recall from our previous lesson videos that there are two types of effective and memory T cells that are responsible for generating an immune response to antigens. And those two main types of T cells are once again the site of toxic T. Cell and the helper T cell. Now the site of toxic T cell is commonly abbreviated as TC cell or C. d. eight cell. And the C. D. A. Is specifically referring to a specific protein that's on the surface of these side A toxic T cells. And so psycho toxic T cell TC cell and CD eight cell are all referring to the same types of cell. And these side a toxic T cells are mainly responsible for releasing chemicals that induce apoptosis of host cells that are infected with intracellular pathogens. And so once again these T cells are targeting intracellular pathogens. Now the helper T cell on the other hand is going to have a different function than the psycho toxic T cell. The helper T cell is going to be commonly abbreviated as A. T. H. Or CD four cells. And again C. D. Four is just the name of a protein that's found on the surface of these helper T cells. And helper T cells. Th cell and CD four cells are all referring to the same type of cell. Now once again the helper T cells are different than side a toxic T cells. They do not have the same functions. The helper T cell as its name implies, is going to help other cells become activated. And so they are going to produce Cida kinds to help stimulate and activate other immune cells. For example helping to stimulate and activate B cells. And again, we'll get to talk more about B cells later in our course after we finished talking about the T cells. And so again the site of toxic T cell can be abbreviated as cd eight cell and the helper T cell can be abbreviated as cD four cell. And that's something that's very important for you to remember. And so again, C. D. Here actually stands for cluster of differentiation. And these are specific markers protein markers that are on the surface of these specific types of cells. And it's one of the ways that scientists can differentiate between the two types of T cells. Uh And so these cD markers are clusters of surface proteins on T cells that again can be used to differentiate side a toxic T cells and helper T cells. Uh So again, the cD markers, the psycho toxic T cells are cD eight and the helper T cells are cD four. And it's the cD markers that can actually dictate or helped to dictate the interactions and the functions of the Cida toxic and helper T cells. And so if we take a look at our image down below, we can further differentiate between the side a toxic and the helper T cell. And so on the left hand side of our image, what we're showing you is the sido toxic T cell. And what you'll notice is that our side a toxic T cell has these cd eight markers on its surface. And again these cd eight markers are going to allow for scientists to um basically differentiate the cida toxic T cell from the helper T cell which we have on the right now the site, a toxic T cell we're showing here also as the grim reaper here because what it does is it can induce apoptosis and host cells that have been affected by an intracellular pathogen. So for example over here we have another host cell that is infected and it is infected with a virus. As you can see right here. And so because this host cell is infected this side a toxic T cell can come and it can induce apoptosis of this infected host cell and that ultimately helps to protect um the entire human body and defend ourselves against these intracellular pathogens. Uh now again, we'll get to talk a lot more details about the functions of Cida toxic T cells as we move forward in our course. Now on the right over here, what we're showing you are the helper T cells and the helper T cells are also known as th cells or cd four cells. And that's because they have Cd four markers or proteins on the surface of these cells And these helper T cells, they are different than the site a toxic T cells. They have a different function. Instead of inducing apoptosis, these helper T cells are going to help activate other cells so they can activate other reinforcements if you will. And so notice over here what we're showing you are a bunch of other immune system cells That can become activated and they become activated by the help of the CD four cells. And so the activated immune cells are going to be capable of generating immune responses and improved immune responses as well. And again, we'll get to talk more details about the functions of helper T cells as we move forward in our course and notice that we're showing you the helper T cell here as a little uh like a crosswalk person that helps people cross the roads and things like that because again it's going to be helping other immune cells to perform their functions better. And so this here concludes our brief introduction to the side, a toxic T cell and helper T cells. And again, we'll be able to apply these concepts as we move forward and learn more about them as well. So, I'll see you all in our next video