The human leukocyte antigen system (HLA) is the name of the major histocompatibility complex (MHC) in humans. HLA class I antigens (A, B & C) present peptides from inside the cell (including viral peptides if present). These peptides are produced from digested proteins that are broken down in the proteasomes. HLA have several roles; They may protect against or allow cancer, and they may mediate autoimmune disease (examples: type I diabetes, celiac disease). Because the HLA antigens are essential to immunity, identification aids in determination of the degree of tissue compatibility between transplant recipients and donors. Testing is done to diminish the likelihood of rejection after transplant, and to avoid graft-versus-host disease (GVHD) following major organ or bone marrow transplantation.