Human leukocyte antigen complex (HLA)

The **human leukocyte antigen complex** is a group of highly polymorphic genes located on chromosome 6 that encode cell-surface glycoproteins essential for regulating the adaptive immune response. These proteins present peptide fragments derived from self and non-self antigens to T lymphocytes—HLA class I molecules present endogenous peptides primarily to cytotoxic CD8+ T cells on all nucleated cells, while class II molecules present exogenous peptides mainly to helper CD4+ T cells on specialized antigen-presenting cells. The diversity generated by numerous alleles allows fine-tuning of pathogen recognition but also underlies susceptibility to various autoimmune diseases and plays a central role in organ transplant compatibility. The term "HLA" is synonymous with the human major histocompatibility complex (MHC). Dysregulation or mismatch can result in autoimmunity or transplant rejection.

Other names

Major histocompatibility complex (MHC) in humansHLA systemLeukocyte antigen complex

Drugs affecting this pathway generally act by suppressing T cell activation or proliferation—either by inhibiting calcineurin signaling (e.g., cyclosporine), blocking co-stimulatory signals required for full T cell activation, or depleting lymphocytes. Monoclonal antibodies may block specific antigens presented via HLA or interfere with their recognition.