MHC (Major histocompatibility complex)

These are protein cluster encoded by genes families which have allelic specificity in every person and responsible for presenting proteins at the surface of cells.
These molecules play a major role in determining whether transplanted tissue will be accepted as self or rejected as foreign.

Class 1 MHC :- 

They contain a large α-chain associated noncovalently with the much smaller β-microglobulin molecules. α-chain is organized into three domain (α₁ α₂ and α₃) each containing 90 Aa, a transmembrane domain of  about 40 Aa and a cytoplasmic anchor segment of 30 Aa.

α₃ domain appears to be highly conserved among class 1 MHC molecules and contain a sequence recognized by CD8 molecule present on T- cells.

Class 1 MHC molecules

Class 2 MHC :- 

They contain different polypeptide chain 33 KDa α and 28 KDa β chain which associate by non-covalent interaction. Each chain contains two external domains α₁ and α₂ and β₁, β₂ domains.
α₂ and β₂ contain a sequence recognized by CD4 molecules.

Class 2 MHC molecules