In this video, we're going to talk more about T cell receptors. First, we need to recall from some of our previous lesson videos that T cells are immune system cells that play a huge role in adaptive cell-mediated immunity. Recall that these T cells 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 TCRs. These T cell receptors or TCRs are 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. Also recall from some of our previous lessons 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.

In terms of the structure of these TCRs, each TCR consists of 2 polypeptide chains, an alpha chain and a beta chain. These 2 polypeptide chains are linked via special types of covalent bonds known as disulfide bonds. Each of these polypeptide chains contains 2 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. The variable region of each of the polypeptide chains, as its name implies, is a region of the polypeptide chain that varies between different TCRs. This variable region is the part that will directly bind to the presented antigen. Again, the TCRs are only capable of binding to presented antigens. They are not capable of binding to free antigens. The amino acid sequence variation in this variable region accounts for the many different types of antigens that different TCRs can bind. 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 toward one very specific antigen.

The constant region is the second region that each of these polypeptide chains contains. The constant region, as its name implies, is a region that remains relatively constant in terms of its amino acid sequence. This constant region is the region that's closest to the cell surface and is very constant in its amino acid sequence. Take a look at our image below, where you can get a better understanding of T cell receptors. On the left-hand side, we are showing you a T cell. Recall that these T cells are going to have thousands of identical T cell receptors. Notice that here we are showing you these T cell receptors that these T cells can have. If we zoom in to this particular T cell receptor, you can see, that this image is showing, a zoom-in of this particular T cell receptor. When we look at the entire T cell receptor, you'll notice that it has those 2 polypeptide chains. It has this alpha chain over here on the left and then it has this beta chain over here on the right. The two chains are linked via disulfide bonds. Notice that each of these chains contains a variable region, which is here at the top. This variable region that you see right here again is going to vary from one TCR to another TCR, vary between different TCRs. Also, it's going to allow for the binding to a presented antigen. Down below, in this green region, what we have is the constant region of these polypeptide chains. The constant region is going to remain relatively constant from one TCR to a different TCR.

This here concludes our brief introduction to these T cell receptors. 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. I'll see you all in our next video.