MHC Class II Structure and Function

These are glycoproteins found on the surface of antigen presenting cells like macrophages, B cells, dendritic cells of the spleen and Langerhans cells of the skin. MHC class II proteins are coded by HLA-D loci on the chromosome 6. These proteins interact with CD4 carrying T-helper cells.

Look at the figure first
Class II MHC Proteins Structure 
The protein is a cell surface protein and a heterodimer consisting of 2 polypeptide chains, α chain (MW: 33,000 D) and β chain (MW: 28,000 D) of approximately equal length joined by non-covalent interactions.

Similar to immunoglobulin, α chain and β chain has 4 regions, constant C terminal regions and hyper variable N-terminal regions comprising of a

a) Cytoplasmic carboxyl region for phosphorylation and cytoskeletal binding

b) A transmembrane region composed of hydrophobic amino acids providing anchorage.

c) A highly conserved α-2 and β-2 domain

d) A highly polymorphic or hypervariable peptide binding N terminal region made of α-1 and β -1 regions.

Functions of MHC class II proteins
  • Helps immune system in recognising self-cells from non-self cells.
  • Helper T cells recognises antigens bound to MHC Class II proteins
  • Responsible for rejection of transplanted organs
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