Major histocompatibility complex class I molecule (for class I); Major histocompatibility complex class II molecule (for class II) (MHC class I (for class I), MHC class II (for class II))

Major histocompatibility complex (MHC) class I and class II molecules are transmembrane glycoproteins central to adaptive immunity. MHC class I molecules are expressed on all nucleated cells, presenting endogenously derived peptides (e.g., viral or tumor antigens) to CD8+ cytotoxic T cells. They consist of a heavy alpha chain (with three extracellular domains and a transmembrane region) and a beta2-microglobulin chain. MHC class II molecules are expressed mainly on antigen-presenting cells, such as dendritic cells, macrophages, and B cells. They present exogenously derived peptides (e.g., from bacteria) to CD4+ helper T cells. Both classes are highly polymorphic, and their genetic variation underlies tissue compatibility (important for transplant success and disease susceptibility). Their core function is to bind peptide fragments and display them on the cell surface for T cell recognition, thereby orchestrating immune responses. Dysfunction and variation in MHC molecules are linked to autoimmune disease, infection susceptibility, cancer progression, and transplantation outcomes.

Other names

HLA class Ihistocompatibility antigen class IHLA class IIhistocompatibility antigen class II

Blocking T cell activation (e.g., anti-MHC antibodies prevent antigen presentation) Modulating peptide loading and presentation (e.g., fusion proteins or peptide blockers) Altering MHC expression (e.g., IFN-γ derivatives upregulate MHC class II molecules) Inducing immune tolerance