Part 7/8:
Antigen processing involves the breakdown of proteins into peptide fragments that are loaded onto MHC molecules for T cell recognition. For endogenous antigens, proteasomes generate peptides presented by MHC I molecules on nearly all nucleated cells, which engage CD8+ T cells.
Exogenous antigens, taken up by phagocytosis or endocytosis, are processed and presented via MHC II molecules on professional APCs, activating CD4+ T cells. Notably, some dendritic cells can perform cross-presentation, where they present exogenous peptides on MHC I molecules, enabling activation of CD8+ T cells even against extracellular antigens.