Major histocompatibility complex class I and class II proteins (MHC class I and II (or MHC-I and MHC-II))

The **major histocompatibility complex (MHC) class I and class II proteins** are large families of polymorphic glycoproteins that play a central role in the adaptive immune system by presenting peptide antigens to T-cells[1][3][6][7]. MHC class I proteins are expressed on all nucleated cells and present short endogenous peptides (typically 8–10 amino acids) to cytotoxic CD8+ T-cells, enabling detection and destruction of infected or malignant cells[1][4][5][6]. MHC class II proteins are mainly expressed on professional antigen-presenting cells such as dendritic cells, macrophages, and B cells, and present exogenous peptides (typically 13–25 amino acids) to CD4+ helper T-cells to orchestrate broader immune responses[3][4][5][7]. Both classes of proteins are highly polymorphic and critical for self–nonself discrimination; mismatched MHC molecules are a major cause of transplant rejection[1][6][7]. While the MHC is not typically considered a direct drug target, it is a key biomarker for disease risk, immune compatibility, and patient stratification in transplantation and immunotherapy.

Other names

Major histocompatibility antigensHLA proteins (in humans)Histocompatibility antigens class I and II

No direct drug mechanism; many drugs suppress T-cell response rather than the MHC directly