The major histocompatibility complex (MHC) is a set of surface proteins located on nucleated cells that plays an important role in distinguishing self from non-self and antigen presentation. There are two types of MHC molecules: class I MHC molecules present antigens to cytotoxic T cells on all nucleated cells, while class II MHC molecules present antigens to helper T cells and are found only on antigen-presenting cells. Both MHC classes consist of a transmembrane alpha chain and beta chain that form a peptide-binding groove to load and present antigen peptides. MHC molecules allow the immune system to recognize foreign substances so the body's immune response can be triggered.