MHC_II_alpha (SM00920) is the alpha domain of class II histocompatibility antigen. MHC_II_beta (SM00921) is the beta domain of class II histocompatibility antigen. Class II MHC glycoproteins are expressed on the surface of antigen-presenting cells (APC), including macrophages, dendritic cells, and B cells. MHC II protein presents extracellular peptide antigens derived from foreign substances such as bacteria. Proteins from pathogens are degraded into peptide fragments within the APC. These fragments are sequestered into endosomes to bind to MHC class II proteins before being transported to the cell surface. MHC class II receptors display antigens for recognition by helper T cells and Inflammatory T cells. CD4+T helper cells recognize MHC class II proteins.