Like their mammalian counterparts, avian MHC Class I molecules, also known as B-F antigens, consist of a highly polymorphic α-chain noncovalently bound to the invariant β2-microglobulin subunit. MHC Class I molecules are expressed on most nucleated cells where they present endogenously synthesized antigenic peptides to CD8+ T lymphocytes which are usually cytotoxic T cells. The monoclonal antibody F21-21 also reacts with turkey β2-microglobulin.