Mouse anti-Human MHC Class II monoclonal antibody for FC, WB, IHC. MHC (major histocompatibility complex) Class II molecules are found only on antigen-presenting cells. The peptides presented by class II molecules are derived from extracellular proteins (not cytosolic as in class I); hence, the MHC class II-dependent pathway of antigen presentation is called the endocytic or exogenous pathway. Loading of class II molecules must still occur inside the cell; extracellular proteins are endocytosed, digested in lysosomes, and bound by the class II MHC molecule prior to the molecule"s migration to the plasma membrane.