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 infected and healthy cells. There are two classes: MHC class I, found on all nucleated cells, presents endogenous antigens to cytotoxic T cells, while MHC class II, present only 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, highlighting the importance of MHCs in adaptive immunity.

Major Histocompatibility Complex Molecules

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, 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. 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 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 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 MHC molecules. And so it turns out that there are 2 main classes of MHC molecules that are conveniently labeled MHC class 1 and MHC class 2 molecules. Now the MHC class 1s are actually found in all nucleated 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 cytotoxic T cells or TC cells. And so that 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. 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 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. And instead of presenting endogenous antigens like MHC class ones, MHC class 2 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 MHC class twos. Only helper T cells will recognize antigens presented by MHC class twos. Now again, these MHC class 2 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 2 molecules are only found on APCs 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 APCs include dendritic cells, macrophages, and B cells, all of which are going to have these MHC class 2 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 these APCs also have MHC class ones. And so there are some cells that 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 CD 8 cells are the cytotoxic T cells and CD 4 cells are the helper T cells. Those CD markers help guide interactions with the correct MHC class. And so this is why CD8 cells or cytotoxic T cells can only interact with MHC class ones, And CD 4 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 MHCs. And so notice up here on the left, what we're showing you is an MHC class 1 molecule. And if we zoom into the membrane right here, what 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 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. An endogenous antigen, resulting originating from the inside of the cell here. It is being presented on these MHC class 1 molecules. And again, 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. And once this cytotoxic T cell, recognizes an antigen presented on MHC class 1, 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 2 molecule. And, what you'll notice is 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. Now in this image, we're showing you a macrophage. And so notice that on its surface, it has this MHC class 2 molecule. And the MHC class 2 molecules are going to be, 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 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 CD4 cell. And, it is capable of recognizing these antigens on MHC class twos and then generating an immune response. And so this here concludes our brief lesson on the 2 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. 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.

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 infected and healthy 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 found only on antigen-presenting cells (APCs) like dendritic cells and macrophages and present exogenous antigens to helper T cells. This interaction is vital for generating appropriate immune responses, highlighting the importance of MHCs in adaptive immunity.

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 mount appropriate responses to different types of pathogens.

How do MHC molecules present antigens to T cells?

MHC molecules present antigens to T cells by displaying peptide fragments on the surface of host cells. 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 receptors (TCRs) on T cells recognize these antigen-MHC complexes, leading to the activation of the T cells and the initiation of an immune response.

What role do antigen-presenting cells (APCs) play in the immune response?

Antigen-presenting cells (APCs) play a crucial role in the immune response by capturing, processing, and presenting antigens to T cells. APCs include dendritic cells, macrophages, and B cells. They express MHC class II molecules, which present exogenous antigens to helper T cells (CD4+). This interaction is essential for the activation of helper T cells, which then coordinate the immune response by activating other immune cells, such as B cells and cytotoxic T cells. APCs also express MHC class I molecules, allowing them to present endogenous antigens to cytotoxic T cells (CD8+), further contributing to the immune defense.

Why are MHC molecules important for adaptive immunity?

MHC molecules are essential for adaptive immunity because they enable the immune system to recognize and respond to specific pathogens. By presenting antigens to T cells, MHC molecules help the immune system distinguish between infected and healthy cells. MHC class I molecules present endogenous antigens to cytotoxic T cells (CD8+), which then target and destroy infected cells. MHC class II molecules present exogenous antigens to helper T cells (CD4+), which activate other immune cells to mount a coordinated response. This antigen presentation is crucial for the specificity and memory of the adaptive immune response, allowing the body to effectively combat infections and remember past encounters with pathogens.