Before we can get into what B and T cells do, we need to talk about how cells display antigens, and this involves proteins called major histocompatibility complex proteins, or just the major histocompatibility complex, or because that's such a mouthful, MHC. That's what I'm going to call it from now on. So, these are just cell surface proteins that will display antigens, and this process is known as antigen presentation, this displaying of antigens at the cell surface using these MHC proteins. So, there's actually going to be 2 classes of MHC proteins that we're going to be concerned with. Class 1 proteins are actually expressed by all cells of the body, and they almost act like a window into the cell; they display antigens that are found inside that cell. This is a way that cells can alert the immune system to an infection inside of that cell. This is also the reason that organ transplants will be rejected by the immune system because the foreign organ will display different MHC 1 proteins. And here, you can sort of see an example of what it might look like, this type of antigen presentation. Some antigens, you know, could be like whole pathogens, will get inside the cell. The MHC class 1 protein inside the cell will bind to it and move to the surface and display it there. Now, MHC class 2 proteins are expressed by antigen-presenting cells. Remember, those are the cells that display antigens that are found in, I'm sorry, those are cells that display antigens that are involved in the adaptive immune system, and those are going to include dendritic cells, the bridge between the adaptive and innate immune systems, macrophages, and B cells. So, these MHC class 2 proteins will display antigens that are actually found outside of the cell, then collected and brought inside the cell. So, these antigen-presenting cells, phagocytes. So they'll phagocytose some antigen, bring it in, bind it to an MHC 2 protein, and then bring that to the cell surface and present that antigen at the membrane. So, dendritic cells do this with a special purpose because they are actually going to grab these antigens and then kind of run to other immune cells to sort of like tattle on them, being like, "Look here, here. Here's a pathogen I found," and they're actually going to not just phagocytose antigens, but actually then degrade them into little fragments, and it's actually going to be those fragments that they display with the MHC class II proteins, so, you know, in this image it looks like it's a whole bacterial cell but in reality, a dendritic cell would just be displaying some fragment from that bacterial cell, some fragment that would act as an antigen.
So with that, let's flip the page.
