ⓘ Antigen processing
Antigen processing, or the cytosolic pathway, is an immunological process that prepares antigens for presentation to special cells of the immune system called T lymphocytes. It is considered to be a stage of antigen presentation pathways. This process involves two distinct pathways for processing of antigens from an organisms own proteins or intracellular pathogens, or from phagocytosed pathogens, subsequent presentation of these antigens on class I or class II major histocompatibility complex molecules is dependent on which pathway is used. Both MHC class I and II are required to bind antigen before they are stably expressed on a cell surface. MHC I antigen presentation typically involves the endogenous pathway of antigen processing, and MHC II antigen presentation involves the exogenous pathway of antigen processing. Cross-presentation involves parts of the exogenous and the endogenous pathways but ultimately involves the latter portion of the endogenous pathway.
When sharing the distinction between these two ways is useful, there are cases when the extracellular-derived peptides in the context of MHC class I and cytosolic peptides presented in the context of MHC class II is often the case in dendritic cells.