Final answer:
The invariant chain, or Ii chain, is the polypeptide chain in MHC Class II molecules that remains constant, assisting in stabilizing MHC Class II molecules before peptide loading and preventing premature binding of peptides.
Step-by-step explanation:
Within the structure of the MHC Class II molecules, the invariant chain (also known as the gamma chain or Ii chain) is the polypeptide chain not variable from one individual to another. It plays a pivotal role in stabilizing the empty MHC Class II molecule before peptides are loaded onto it.
The MHC Class II molecules are key to the immune system, as they present antigens derived from extracellular proteins to T cells.
Unlike the highly polymorphic alpha (a) and beta (B) chains that form the antigen-binding groove, the invariant chain’s primary function is to block the binding groove of the MHC Class II molecule while in the endoplasmic reticulum to prevent it from binding peptides prematurely.
It also aids in the transport of MHC Class II molecules to the endosomal/lysosomal compartments where peptide loading occurs