Learning about the antigen presenting pathways, and I am confused on the Endogenous, exogenous and cross presentation. I through endogenous was peptides in cell, and exogenous was peptides outside cell (peptides from pathogens), but the protein (in exogenous pathway) first enters the cell via endocytosis, and then is broken down, binds to MHC class 2 and then goes to cell surface and is expressed. So then what’s the difference here??? Why the different naming, and different MHC molecules if the protein has to enter the cell anyways?
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I believe it’s because exogenous means to come “from outside the body” where endogenous is “inside the body.” For example exogenous testosterone administration would be taking a shot of testosterone whereas in a male, their testicles producing testosterone would be endogenous production. So I believe since it is inside the body, it is still endogenous even if it’s from outside the cell. I could be wrong but that’s how I’ve always heard endogenous vs exogenous