before we can get into what B and T cells do, we need to talk about how cells display antigens. And this involves these proteins called major history compatibility, complex proteins or just the major history compatibility complex. Or because that's such a mouthful, MHC. That's what I'm going to call it from now on. So these were just cell surface proteins that will display antigens. And, uh, this process is known as Auntie Jen Presentation this displaying of antigens at the cell surface using these MHC proteins. So there's actually gonna be two classes of MHC proteins that we're gonna be concerned with. Class one are actually expressed by all cells of the body, and these almost act like a window into the cell. They display antigens that air found inside that cell. So this is a way that cells can alert the immune system thio an infection inside of that cell. Uh, this is also the reason that organ transplants, uh, will be rejected by the immune system because the foreign Oregon will display different MHC one proteins. And here you can sort of see unexamined alot of what it might look like. This type of antigen presentation so some anti agent could be like whole pathogen will get inside the cell. The MHC class one protein inside the cell will bind to it and moved to the surface and display it there. Now MHC class two proteins are expressed by antigen presenting cells. Remember those air the cells that display antigens that air found? And, uh, I'm sorry, those air 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 system, macrophages and B cells. So these MHC class two proteins will display antigens that air actually found outside of the cell, then collected and brought inside the cell. So these antigen presenting cells remember dendritic cells and macrophages are faggots sites. So they'll faggot faggot Saito's some Auntie Jen, bring it in, bind it to an MHC to protein, and then bring that to the cell surface and present that Auntie Jen at the membrane. So dendritic cells do this with a special purpose because they are actually going thio, grab these antigens and then kind of run thio other immune cells to sort of like tattle on them being like, Look here, here's Ah, pathogen, I found. And they're actually going Thio not just faggots Ito's antigens, but actually then degrade them into uhh, you know, little fragments. And it's actually gonna be those fragments that they display with the MHC Class two proteins. So, you know, in this image, it looks like it's a whole bacterial cell. But in reality, you know, a dendritic cell would just be displaying some fragment from that bacterial cell, some fragment that would act as an anti gin. So with that, let's flip the page.