Final answer:
MHC Class II molecules are made up of two subunits, alpha and beta, and have a cleft that serves as the antigen-binding site near the top of the molecule.
Step-by-step explanation:
MHC Class II molecules are made up of two subunits, an alpha (α) chain and a beta (β) chain, which are approximately similar in length. Each chain folds into two separate domains, alpha 1 and alpha 2, and beta 1 and beta 2. The MHC Class II molecule has a cleft that serves as the antigen-binding site near the outermost portion of the molecule. In order to present abnormal or non-self-antigens to T cells, MHC molecules have a cleft that serves as the antigen-binding site near the 'top' portion of the MHC-II dimer. The cleft in MHC II is formed by the alpha 1 and beta 1 domains.