The human leukocyte antigen (HLA) system is a gene complex encoding the major histocompatibility complex (MHC) proteins in humans. These cell-surface proteins are responsible for the regulation of antigen-specific immune responses in humans. HLA genes are highly polymorphic, which enables them to fine-tune the adaptive immune response. The class I HLA molecules (HLA-A, -B, and -C) present small peptide antigens, approximately 8 to 10 amino acids in length, originating from inside the cell to CD8+ cytotoxic T lymphocytes, thereby activating a cytotoxic T-cell response. The class II HLA molecules (HLA-DP, -DM, -DO, -DQ, and -DR) present antigens derived from outside the cell, approximately 15 to 24 residues in length, to CD4+ helper T cells. These helper T cells subsequently secrete cytokines and stimulate B cells to produce antibodies specific to that antigen. The class III HLA region encodes components of the complement system.
