Major Histocompatibility Complex Molecules

in this video we're going to begin our lesson on major historic compatibility complexes. But before we define what those are. Its first helpful to note that immune cells must properly identify threats and only target infected host cells and pathogens which are dangerous to us. However, our immune cells should not be targeting uninfected or healthy host cells that are harmless to us. And so one of the ways that T cells can detect threats is by interacting with really special molecules on our host cells that are called em H. C. S. And so these M. H. C. S. R. It's really just an abbreviation for major historic compatibility complexes. And these major historic compatibility complexes or MHC s can be defined as surface proteins on our host cells that present antigens to T cells. And then those T cells can then then generate an appropriate immune response. Now if we take a look at our image down below which will notice is that on the left hand side over here we're showing you a macrophage which is an example of one of our own host cells. And notice that on the surface of our macrophage are these little molecules here and this little molecule that you see right here is a major historic compatibility complex or in other words and M. H. C. Now notice that these MHC is what they do is they present antigens. And so notice here these are being labeled as antigens and they are presenting antigens to these T cells. And over here on the right what we have is a helper T cell A th cell. And remember that this is also known as a cd four cell. And uh of course this helper cell can use its T. C. R. Or T cell receptor in order to detect the MHC presenting an antigen. And of course that will allow for the helper T cell to generate an appropriate immune response, which we'll get to talk more details about as we move forward in our course. Now it does turn out that there are two major types of major historic compatibility complexes and we'll be able to talk about those two major types as we go forward. So I'll see you all in our next video.

in this video, we're going to begin our lesson on the classes of MHC molecules. And so it turns out that there are two main classes of MHC molecules that are conveniently labeled MHC Class one And MHC Class two molecules. Now the MHC Class ones are actually found in all nuclear hated host cells. And so that means that MHC class ones are found in pretty much all of our cells. And so these MHC class ones, they actually present endogenous antigens and they're only going to be recognized by Cida toxic T cells or T C cells. And so that means that helper T cells do not recognize antigens presented on MHC Class ones. Only side a toxic T cells recognize antigens presented on MHC class Once now again, these MHC class ones, they present endogenous antigens. And so these endogenous antigens can be defined as antigens that originate from inside of the cell. For example, an antigen that belongs to a virus, a viral antigen where the virus has made its way inside of the cell. And those viral antigens can be presented on MHC class ones. Now, MHC Class two molecules on the other hand, are not found on all nuclear ated host cells. MHC class two molecules are only found in some of our host cells and instead of presenting endogenous antigens like MHC class ones, MHC class two molecules present xO genesis antigens and they're only going to be recognized by helper T cells or th cells. And so that means that Cida toxic T cells will not recognize antigens presented on MHC Class two only helper T cells will recognize antigens presented by MHC Class twos. Now again, these MHC Class two molecules they present exogenous antigens. And so these exogenous antigens can be defined as antigens that originate from outside of the host cell. And also these MHC Class twos. Again, they're only found in some host cells. And more specifically, these MHC Class two molecules are only found on a P. C. S. Or antigen presenting cells which we covered a little bit in some of our previous lesson videos. And so recall that the antigen presenting cells or a pcs include dendritic cells, macrophages and B cells, all of which are going to have these MHC Class two molecules. Now again, these dendritic cells, macrophages and B cells, it turns out that they are also nuclear gated cells. And so because they are nuclear gated cells, that means that the these ap CS also have MHC class ones. And so there are some cells that have both MHC Class one and MHC Class two. But again, MHC Class two are only found on the A. P. C. S. Now what you'll notice is that the C. D markers that are found on T cells which recall Cd eight cells or the site a toxic T cells and cd four cells are the helper T cells, those cd markers help guide interactions with the correct MHC class. And so this is why Cd eight cells or psycho toxic T cells can only interact with MHC Class ones and CD four cells or helper T cells can only interact with MHC Class twos. And so if we take a look at our image down below, we can get a better understanding of these two major classes of MHC s and so notice up here on the left, what we're showing you is an MHC Class one molecule. And if we zoom into the membrane right here, which you'll see is that this is just showing you a nuclear hated host cell and notice that it is infected. This is an infected host cell because on the inside you'll notice that there is a virus that is replicating. And so what can happen is that these viral antigens, this endogenous antigen which we can label right here, this is an endogenous let's label this in blue or green actually. And endogenous antigen resulting originating from the inside of the cell here. Uh it is being presented on these MHC Class one molecules. And again, it's the site a toxic T cell or this TC cell or CD eight cell that is going to be able to recognize antigens presented on MHC class one molecules. And once this side a toxic T cell recognizes an antigen presented on MHC class one, it can then carry out an immune response. And again, we'll talk about those immune responses as we move forward Now down below what we're showing you is the MHC class two molecule and which will notice is that the MHC Class two molecules are only found on very specific host cells, mainly the Ap CS, which again include dendritic cells, macrophages and B cells. Now in this image we're showing you a macrophage And so notice that on its surface it has this MHC class two molecule and the MHC Class two molecules are going to be um presenting exogenous antigens or antigens that are going to originate from the outside of the host cell. And as we'll learn later in our course, these exogenous antigens will be internalized and then once they're internalized they will be presented back on the surface on these MHC Class twos. And again the antigen will originate from the outside of the cell and is causing harm on the outside of the cell. And so what you'll see here is that these antigens presented on MHC Class two can only be recognized by helper T cells. And so over here on the right, what we're showing you is a th cell or a helper T cell, also known as a Cd four cell. And it is capable of recognizing these antigens on MHC class 2s and then generating an immune response. And so this here concludes our brief lesson on the two major classes of MHC molecules. MHC class ones found on all nuclear ated host cells presenting endogenous antigens that only cite a toxic T cells can recognize and then MHC Class two molecules which are going to be only found on some host cells, mainly these ap CS presenting exogenous antigens that only helper T cells can recognize. And so we'll be able to get some practice applying these concepts as we move forward and continue to learn more as well. So I'll see you all in our next video.