The major histocompatibility complex (MHC) is a set of cell-surface proteins crucial for adaptive immunity, presenting peptide fragments to T lymphocytes. Class I molecules, expressed on almost all nucleated cells, display endogenous peptides to CD8+ cytotoxic T cells; Class II molecules, on antigen-presenting cells, present exogenous peptides to CD4+ helper T cells. MHC genes are highly polymorphic, affecting transplant compatibility and disease susceptibility.

MHC class I molecules present intracellular endogenous peptides on nearly all nucleated cells to CD8+ cytotoxic T cells, enabling recognition and killing of virus-infected or abnormal cells. MHC class II molecules, expressed mainly on professional antigen-presenting cells (dendritic cells, macrophages, B cells), present extracellular exogenous peptides to CD4+ helper T cells to activate and coordinate adaptive immune responses, including antibody production.

The major histocompatibility complex (MHC) has two key functions: presenting peptide antigens to T lymphocytes—MHC I displays endogenous (intracellular) peptides to CD8+ cytotoxic T cells, while MHC II presents exogenous (extracellular) peptides to CD4+ helper T cells—and mediating self versus non‑self discrimination, guiding T‑cell selection and immune tolerance to prevent autoimmunity.

George Snell is widely regarded as the father of the major histocompatibility complex (MHC). He discovered and mapped the mouse H‑2 histocompatibility complex, demonstrating genetically determined cell-surface molecules that control tissue rejection. His work laid the foundation for transplantation genetics and modern immunology and earned him a share of the 1980 Nobel Prize in Physiology or Medicine.

The major histocompatibility complex (MHC) displays peptide fragments on cell surfaces for T‑cell recognition, distinguishing self from non‑self. MHC class I presents intracellular (endogenous) peptides to CD8+ cytotoxic T cells; MHC class II presents extracellular (exogenous) peptides to CD4+ helper T cells via antigen-presenting cells. MHC polymorphism influences pathogen response and transplant compatibility, making it central to adaptive immunity.

MHC class I molecules present peptide fragments from intracellular proteins on the surface of nearly all nucleated cells. Their main function is to display endogenous antigens to CD8+ cytotoxic T lymphocytes, enabling recognition and elimination of virus-infected or malignant cells and helping maintain immune surveillance and self-tolerance. They are essential for adaptive cellular immunity.