<prompt> Which poion of MHC 1 complex forms the component of antigen presenting pa:Between alphal-beta 2 microglobulinDistal pa of alpha chainProximal pa of alpha chainBetween as and b2 microglobulin  <response>: Ans. b Distal pa of alpha chain Ref. Ananthanarayan 8e, p 133, 9e p 139 MHC I HLA I - Class 1 molecule consists of a three domains of heavy alpha peptide chain al; a2; a3 which are non-covalently linked to smaller B2 microglobulin peptide. - The distal domain alpha 1 and alpha 2 of MHC 1 have highly variable amino acid sequences and are folded to form a cavity or groove. Protein antigens are bound to this groove for presentation to CD8 T cells. MHC II HLA II - MHC II antigens are heterodimer consisting of an alpha and beta chain. Each chain has two domain proximal one is constant and distal one is variable. - The two distal domain alpha 1 , beta 1 constitute the antigen binding site, for recognition by CD4 T cells. - Both class 1 and class 2 are members of immunoglobu I in gene super family. MHC III HLA III - MHC III are heterogenous they include complement components involved in the formation of C3 convease, heat shock protein and tumor necrosis factor. 