Major Histocompatibility Complex (MHC)Gene Products Class I Antigen is usually endogenous (e.g.viral proteins). CD8+Cytotoxic T Lymphocytes (CTLs)recognize antigen in association with class I MHC gene product on APC. Class II Molecules Antigen is usually extracellular. CD4+Helper T Lymphocytes recognize antigen in association with class II MHC gene product on APC. Major Histocompatibility Complex (MHC) Gene Products ◼ Class I ◼ Antigen is usually endogenous (e.g. viral proteins). ◼ CD8+ Cytotoxic T Lymphocytes (CTLs) recognize antigen in association with class I MHC gene product on APC. ◼ Class II Molecules ◼ Antigen is usually extracellular. ◼ CD4+ Helper T Lymphocytes recognize antigen in association with class II MHC gene product on APC