Ans. is 'a' i.e., All nucleated cells o The histocompatibility antigens (human leukocyte antigens - HLA) are cell surface antigens that induce an immune response leading to rejection of allografts. The principal physiologic function of the cell surface histocompatibility molecules is to bind peptide fragments offoreign proteins for presentation to antigen specific T cells.o The histocompatibility antigens are encoded by a closely linked multiallelic cluster of genes - Major histocompatibility complex (MHC) or Human leukocyte antigens complex (HLA complex). HLA complex of genes is located on the short arm of chromosome 6.o HLA complex consists of three separate clusters of genes :-1) Class I (MHC-I)# It comprising of A,B and C loci (i.e. HLA-A, HLA-B, HLA-C). It is found on the surface of all nucleated cells and platelets.# Important cells with MHC-I (HLA-I) on surface are B-cells, T-cells, macrophages/monocytes, neutrophils, langerhans cells, dendritic cells, platelets (thrombocytes), epithelial cells of thymus and hepatocytes. MHC class I present antigen to cytotoxic CD-8 T cells (MHC-I restricted).# MHC class I is responsible for graft rejection and cell mediated cytolysis of viral infected or tumor cells.2) Class II (MHC-II)# It comprises *D ' region (HLA-DR, HLA-DQ, HLADP). It is found only on the cells of immune system, i.e. T- cells, B-cells , langerhans cells, dendritic cells, and macrophages.# MHC-II recognizes exogenous antigen (extracellular microbes, soluble proteins) and present it to CD4 helper T cells (MHC-II restricted). MHC-II is responsible for graft versus host response and mixed leukocyte reaction.3) Class III(MHC-III)# It encodes for complement components C2 & C4, properdin factor B, Heat shock protein and Tumor necrosis factor-a and b (TNF-a and b). MHC-III is involved in susceptibility to autoimmune diseases like SLE.RBCs do no have MHC molecule