Major Histocompatibility Complex: MHC I vs MHC II

Quick explanation of MHC I and II since I've had a lot of questions on this recently :)  MHC I is the class expressed on all nucleated cells (not RBCs) and basically says to the immune system "Hi friend I belong here". It represents cytosolic fragments. It is related to cytotoxic T cells because these are the cells that will help you fight a viral infection. If a virus gets inside the cell it will no longer be saying the right "Hi I'm me" message through MHC I. It'll be like "Hi I'm me with a side of virus".


Now on the other hand MHC II presents things that were taken into the cell by phagocytosis etc so it will be useful for bacterial infections. Thus it is related to your T helper cells so your immune system can clear up the bacteria. MHC II is located on all your antigen presenting cells such as macrophages and dendritic cells. Hopefully this quick picture will help you remember the difference. If you get asked a question about what type of MHC are on bladder cells or some random area of the body don't freak out! You didn't miss any histology notes, and it's not a trick question. If it's just a regular cell it's going to present MHC I. So unless the questions is asking about a macrophage or other antigen presenting cell just stay calm and remember all those bladder cells have MHC I.

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