Major Histocompatibility Complex Molecules - Online Tutor, Practice Problems & Exam Prep

Major histocompatibility complexes (MHCs) are crucial surface proteins on host cells that present antigens to T cells, enabling the immune system to distinguish between healthy and infected cells. There are two classes: MHC class I, found on all nucleated cells, presents endogenous antigens to cytotoxic T cells, while MHC class II, located on antigen-presenting cells (APCs) like dendritic cells and macrophages, presents exogenous antigens to helper T cells. This interaction is vital for generating appropriate immune responses against pathogens.

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 MHCs. And so these MHCs are, is really just an abbreviation for major histocompatibility complexes. And these major histocompatibility complexes or MHCs can be defined as surface proteins on our host cells that present antigens to T cells, and then those T cells can then generate an appropriate immune response.

Now, if we take a look at our image down below, what 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 host cells. 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 MHC. Now notice that these MHCs, 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 CD4 cell. And, of course, this helper cell can use its TCR 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 2 major types of major histocompatibility complexes, and we'll be able to talk about those 2 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 MHC molecules. It turns out that there are two main classes of MHC molecules that are conveniently labeled MHC class 1 and MHC class 2 molecules. The MHC class 1s are actually found in all nucleated host cells, which means that MHC class ones are found in pretty much all of our cells. These MHC class ones actually present endogenous antigens and they're only going to be recognized by cytotoxic T cells or TC cells. This means that helper T cells do not recognize antigens presented on MHC class 1s. Only cytotoxic t cells recognize antigens presented on MHC class ones. These MHC class ones present endogenous antigens, which can be defined as antigens originating from inside of the cell. For example, 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 2 molecules, on the other hand, are not found on all nucleated host cells. MHC class 2 molecules are only found in some of our host cells. Instead of presenting endogenous antigens like MHC class ones, MHC class 2 molecules present exogenous antigens. They're only going to be recognized by helper T cells or TH cells. This means that cytotoxic T cells will not recognize antigens presented on MHC class twos. Only helper T cells will recognize antigens presented by MHC class twos. Again, these MHC class 2 molecules present exogenous antigens, defined as antigens originating from outside of the host cell. Also, these MHC class twos are only found in some host cells and more specifically, they are only found on APCs or antigen-presenting cells, which include dendritic cells, macrophages, and B cells, all of which are going to have these MHC class 2 molecules. These dendritic cells, macrophages, and B cells are also nucleated cells. Because they are nucleated cells, these APCs also have MHC class ones. Some cells have both MHC class 1 and MHC class 2. But again, MHC class 2 are only found on the APCs.

Now, what you'll notice is that the CD markers that are found on T cells, which recall CD8 cells are the cytotoxic T cells and CD4 cells are the helper T cells. Those CD markers help guide interactions with the correct MHC class. This is why CD8 cells or cytotoxic T cells can only interact with MHC class ones, and CD4 cells or helper T cells can only interact with MHC class twos.

If we take a look at our image down below, we can get a better understanding of these two major classes of MHCs. Notice on the left, what we're showing you is an MHC class 1 molecule. If we zoom into the membrane here, you'll see this is just showing a nucleated host cell. Notice that it is an infected host cell because on the inside, there is a virus that is replicating. These viral antigens, this endogenous antigen, is being presented on these MHC class 1 molecules. It's the cytotoxic T cell, or this TC cell or CD8 cell that is going to be able to recognize antigens presented on MHC class 1 molecules. Once this cytotoxic T cell recognizes an antigen presented on MHC class 1, it can then carry out an immune response. We'll talk about those immune responses as we move forward.

Below, what we're showing you is the MHC class 2 molecule. You'll notice that the MHC class 2 molecules are only found on very specific host cells, mainly the APCs, which again include dendritic cells, macrophages, and B cells. In this image, we're showing you a macrophage. Notice that on its surface, it has this MHC class 2 molecule. The MHC class 2 molecules present exogenous antigens or antigens that originate from outside of the host cell. Once internalized, they will be presented back on the surface on these MHC class twos. These antigens presented on MHC class twos can only be recognized by helper T cells. Over on the right, what we're showing you is a TH cell or a helper T cell also known as a CD4 cell. It is capable of recognizing these antigens on MHC class twos and then generating an immune response.

This concludes our brief lesson on the two major classes of MHC molecules: MHC class ones, found on all nucleated host cells presenting endogenous antigens that only cytotoxic T cells can recognize, and then MHC class 2 molecules, which are going to be only found on some host cells mainly these APCs presenting exogenous antigens that only helper T cells can recognize. 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.

Here’s what students ask on this topic:

What are major histocompatibility complex (MHC) molecules?

Major histocompatibility complex (MHC) molecules are surface proteins found on host cells that play a crucial role in the immune system. They present antigens to T cells, enabling the immune system to distinguish between healthy and infected cells. There are two main classes of MHC molecules: MHC class I and MHC class II. MHC class I molecules are found on all nucleated cells and present endogenous antigens to cytotoxic T cells. MHC class II molecules are located on antigen-presenting cells (APCs) like dendritic cells and macrophages and present exogenous antigens to helper T cells. This interaction is essential for generating appropriate immune responses against pathogens.

What is the difference between MHC class I and MHC class II molecules?

MHC class I and MHC class II molecules differ in their distribution, the type of antigens they present, and the T cells they interact with. MHC class I molecules are found on all nucleated cells and present endogenous antigens, which originate from inside the cell, to cytotoxic T cells (CD8+). In contrast, MHC class II molecules are found only on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. They present exogenous antigens, which originate from outside the cell, to helper T cells (CD4+). This distinction is crucial for the immune system to target infected cells and pathogens effectively.

How do MHC molecules present antigens to T cells?

MHC molecules present antigens to T cells by displaying peptide fragments on the cell surface. MHC class I molecules present endogenous antigens, derived from proteins within the cell, to cytotoxic T cells (CD8+). These antigens are typically from intracellular pathogens like viruses. MHC class II molecules present exogenous antigens, derived from extracellular sources, to helper T cells (CD4+). These antigens are usually from extracellular pathogens like bacteria. The T cell receptor (TCR) on T cells recognizes the antigen-MHC complex, leading to the activation of the T cell and the initiation of an immune response.

What types of cells express MHC class II molecules?

MHC class II molecules are expressed on antigen-presenting cells (APCs), which include dendritic cells, macrophages, and B cells. These cells are specialized in capturing, processing, and presenting exogenous antigens to helper T cells (CD4+). Unlike MHC class I molecules, which are found on all nucleated cells, MHC class II molecules are restricted to these specific immune cells. This selective expression allows for a targeted immune response, as helper T cells can recognize and respond to antigens presented by APCs, leading to the activation of other immune cells and the coordination of the immune response.

Why are MHC molecules important for the immune system?

MHC molecules are essential for the immune system because they enable the recognition and differentiation of self from non-self. By presenting antigens to T cells, MHC molecules help the immune system identify and target infected or abnormal cells while sparing healthy cells. MHC class I molecules present endogenous antigens to cytotoxic T cells, which can then destroy infected cells. MHC class II molecules present exogenous antigens to helper T cells, which coordinate the activation of other immune cells. This antigen presentation is crucial for initiating and regulating immune responses, ensuring that the body can effectively combat infections and diseases.