Major histocompatibility complex class II antigen (MHC class II)

Major histocompatibility complex class II antigen is a membrane-bound, heterodimeric glycoprotein receptor expressed primarily on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B lymphocytes. It is comprised of separate alpha and beta chains encoded by multiple loci (HLA-DR, -DP, -DQ in humans). MHC class II molecules present peptide fragments, typically 13–25 amino acids in length, derived from extracellular proteins that have undergone endocytic processing. These peptides are bound in a groove formed by the α and β subunits and displayed on the cell surface for recognition by CD4+ T lymphocytes, initiating adaptive immune responses, including cytokine production and B cell help. Peptide loading is controlled by the invariant chain (Ii or CD74) and further edited by HLA-DM and -DO, enabling tight regulation and specificity in antigen presentation. Polymorphisms in MHC class II loci are associated with diverse immune-mediated conditions and transplantation outcomes. MHC class II antigens are fundamental for immune surveillance, pathogen recognition, and immunological memory, and their expression and function are at the center of immunotherapeutic and diagnostic strategies across infection, autoimmunity, transplantation, and cancer.

Other names

MHC class IIHuman leukocyte antigen class II (HLA class II)HLA-DR, HLA-DQ, HLA-DP (human isotypes)Class II major histocompatibility antigenMHC-IIHistocompatibility antigen class II

Modulation of antigen presentation (enhancing or inhibiting peptide loading, editing); Blocking T cell activation (immunosuppression via APC targeting); Immune checkpoint modulation; Editing peptide occupancy (e.g., HLA-DM enhancement or inhibition)