Class I histocompatibility antigens, also known as major histocompatibility complex class I (MHC-I) antigens, are cellular surface proteins found in vertebrates, including humans, that play a crucial role in the immune system. These antigens are encoded by genes located within the major histocompatibility complex (MHC) region on chromosome 6 in humans.
The primary function of class I histocompatibility antigens is to present intracellular peptides to cytotoxic T lymphocytes (CTLs), a type of immune cell that recognizes and eliminates infected or abnormal cells. By binding to specific peptide fragments derived from intracellular proteins, class I antigens present these peptides on the cell surface, allowing CTLs to recognize and target cells that display foreign or abnormal peptides. This process is essential for immune surveillance and defense against viral infections, tumors, and other pathogens.
Class I histocompatibility antigens consist of a polymorphic alpha chain, encoded by the MHC-I genes, and a non-polymorphic beta-2 microglobulin (β2M) chain. Together, they form a heterodimeric complex that is expressed on the surface of nearly all nucleated cells in the body. The alpha chain consists of three extracellular domains: α1, α2, and α3. The α1 and α2 domains form a peptide-binding cleft where the peptide fragments are captured and presented to CTLs. The α3 domain, along with β2M, interacts with specific receptors on CTLs, initiating an immune response.
The polymorphic nature of class I histocompatibility antigens is critical for the diversity of peptide presentation and immune response. Due to extensive genetic variation within the MHC region, individuals express a wide array of different class I alleles, leading to diverse
