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Class I antigen Expressed on all nucleated cells and platelets Encode cell surface glycoproteins Encoded by 3 closely related loci HLA-A, HLA-B & HLA-C CD8+ T cells widely express class I HLA antigen Class II antigen Expressed on macrophages, B cells and T cells Encode cell surface glycoproteins Have a single loci HLA-D (with 3 surface loci DR,DQ,DP) Class III antigen Encodes complements of the complement system Genes coding for histocompatibility antigens are clustered on a small segment of chromosome 6. This cluster constitutes human MHC or HLA complex The class III molecules loci , are present in between the loci for class I and class II molecules (ref: Harrison's 18/e p2685-2687)