Major Histocompatibility Complex Molecules - Video Tutorials & Practice Problems

Major Histocompatibility Complexes

In this video, we're going to begin our lesson on major histocompatibility complexes. But before we define what those are, it's 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 M H CS. And so these MH CS are uh is really just an abbreviation for major histocompatibility complexes. And these major histocompatibility complexes or MH CS can be defined as surface proteins on our host cells that present antigens to T cells. And then those T cells can then uh then generate inappropriate immune response. Now, if we take a look at our image down below which you'll 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 uh 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 histocompatibility complex. Or in other words, an MH C. Now notice that these Mh CS, what they do is they present antigens. And so notice here these are being labeled as antigens and they are uh presenting antigens to these uh T cells. And over here on the right, what we have is a helper T cell, a TH cell. And uh remember that this is also known as a CD four cell. And uh of course, this helper cell can use its TCR or T cell receptor in order to uh detect the MH C presenting an antigen. And uh 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 histocompatibility 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.

Classes of MHC Molecules

In this video, we're going to begin our lesson on the classes of MH C molecules. And so it turns out that there are two main classes of Mh C molecules that are conveniently labeled Mh C class one and Mh C class two molecules. Now, the MH C class ones are actually found in all nucleated host cells. And so that means that Mh C class ones are found in pretty much all of our cells. And so these Mh C class ones, they actually present endogenous antigens and they're only going to be recognized by cytotoxic T cells or TC cells. And so that means that helper T cells do not recognize antigens presented on MH C class ones, only cytotoxic T cells recognize antigens presented on MH C class ones. Now, again, these Mh C 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 uh belongs to a virus, a viral antigen where the virus has made its way inside of the cell. And uh those viral antigens can be presented on MH C class ones. Now, Mh C class two molecules. On the other hand, are not found on all nucleated host cells. Mh C class two molecules are only found in some of our host cells and instead of presenting endogenous antigens like Mh C class ones, Mh C class two molecules present exogenous antigens and they're only going to be recognized by helper T cells or th cells. And so that means that cytotoxic T cells will not recognize antigens presented on MH C class twos, only helper T cells will recognize antigens presented by MH C class twos. Now again, these Mh C 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 Mh C class twos. Again, they're only found in some host cells. And more specifically, these Mh C class two molecules are only found on A P CS or antigen presenting cells, which we covered a little bit in some of our previous less of videos. And so recall that the antigen presenting cells or A P CS include dendritic cells, macrophages and B cells, all of which are going to have these Mh C class two molecules. Now again, these dendritic cells, macrophages and B cells, it turns out that they are also nucleated cells. And so because they are nucleated cells, that means that the uh these A P CS also have MH C class ones. And so there are some cells that have both MH C class one and MH C class two. But again, Mh C class two are only found on the A P CS. Now, uh what you'll notice is that the CD markers that are found on T cells, which recall CD eight cells are the cytotoxic T cells and CD four cells are the helper T cells. Those CD markers help guide interactions with the correct MH C class. And so this is why CD eight cells or cytotoxic T cells can only interact with MH C class ones and CD four cells or helper T cells can only interact with MH C 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 Mh CS. And so notice up here on the left, what we're showing you is an MH C class one molecule. And if we zoom into the membrane right here, which you'll see is that this is just showing you a nucleated 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 replicated. And so what can happen is that these viral antigens uh this endogenous antigen uh which we can label right here. This is an endogenous, let's label this in blue or green, actually an endogenous antigen uh resulting originating from the inside of the cell here. Uh it is being presented on these MH C class one molecules. And again, it's the cytotoxic T cell uh or this TC cell or CD eight cell that is going to be able to recognize antigens presented on MH C class one molecules. And once this cytotoxic T cell uh recognizes an antigen presented on an MH C 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 MH C class two molecule. And uh what you'll notice is that the MH C class two molecules are only found on very specific host cells. Uh mainly the A P 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 Mh C class two molecule and the MH C 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 uh the surface on these MH C class twos. And uh again, uh the antigen will originate from the outside of the cell and is causing harm on the outside of the cell. And so, uh what you'll see here is that these antigens presented on MH C class twos 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 uh it is capable of recognizing these antigens on MH C class twos and then generating an immune response. And so this here concludes our brief lesson on the two major classes of MH C molecules. Mh C class ones found on all nucleated host cells presenting endogenous antigens that only cytotoxic T cells can recognize. And then M AC class two molecules which are going to be only found on some host cells, mainly these A P 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.

Which markers are found on all nucleated cells?

• A. MHC class I.
• B. MHC class II.
• C. CD4.
• D. CD8.

MHC class II molecules are found on:

• A. Basophils & macrophages.
• B. B cells & Neutrophils.
• C. Macrophages & Dendritic cells.
• D. T helper cells & Macrophages.
• E. All antigen presenting cells (APCs).

Once an MHC II molecule on an APC presents an antigen:

• A. Effector cells with CD8 markers activate the APC.
• B. Effector cells with CD4 kill the APC.
• C. Effector cells with CD8 kill the APC.
• D. Effector cells with CD4 activate the APC.

If an effector CD4 cell encounters an antigen presented on a MHC Class II molecule, how would it respond?

• A. Send signals to the APC trigger apoptosis in the APC.
• B. Engulf the APC and degrade the infected cell and the intracellular pathogen.
• C. Send signals to the APC, activating the APC and triggering an immune response.
• D. All of the following are possible responses of the effector CD4 cell.