Antigen processing 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 organism's own (self) proteins or
intracellular pathogens (e.g.
viruses), or from
phagocytosed pathogens (e.g.
bacteria); subsequent presentation of these antigens on class I or class II MHC 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 (considering
cross-presentation) 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 (e.g. proteolysis of antigens for binding to MHC I molecules).