ADAPTIVE IMMUNE RESPONSES Antigen Processing Antigen-presenting cells (APCs) encounter native antigens, endocyte and process them. A subsequent event, processing, implies the disassembly of native antigens and complexation of single epitopes with Class I and Class II HLA molecules inside the antigen-presenting cell. The processed antigen becomes an immunogen, i.e., an immunologically active substance. There are two pathways of antigen processing, depending on the antigen type: HLA II pathway for exogenous antigens and HLA I pathway for endogenous, including intracellularly disposed microbe-derived antigens such as viral antigens. ©V.V.Klimov