Component of the class I major histocompatibility complex (MHC). Involved in the presentation of peptide antigens to the immune system (By similarity). Heterodimer of an alpha chain and a beta chain. Beta-2-microglobulin is the beta-chain of major histocompatibility complex class I molecules. Forms a heterotrimer with MR1 and a metabolite antigen. Belongs to the beta-2-microglobulin family.