Major histocompatibility complex class I and class II molecule (MHC class I and II)

Major histocompatibility complex (MHC) class I and class II molecules are highly polymorphic, membrane-bound glycoproteins central to adaptive immunity[1][4][5][6]. **MHC class I molecules** are expressed on nearly all nucleated cells and present peptide fragments derived from intracellular proteins to CD8+ cytotoxic T cells, thereby supporting immune surveillance against viruses, intracellular bacteria, and transformed (cancerous) cells[1][3][4][6]. Their structure includes a polymorphic heavy alpha chain (with α1, α2, α3 domains) noncovalently linked to invariant beta2-microglobulin[5][6]. **MHC class II molecules** are mainly expressed on antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells, and present extracellularly derived peptides to CD4+ helper T cells, modulating immune coordination and response[1][3][4][5][6]. Structurally, they include two chains (α and β), each with two extracellular domains, forming a peptide-binding groove[5]. Variation in MHC molecules underlies differences in immune responses, disease susceptibility, transplant compatibility, and response to immunotherapies[7]. Because of their key antigen-presenting role, MHC molecules are central therapeutic, diagnostic, and biomarker targets in immunology, oncology, infectious disease, and transplantation medicine[4][6].

Other names

HLA class IHLA class IIMHC-IMHC-IIHuman leukocyte antigen class IHuman leukocyte antigen class II

Modulation/suppression of antigen presentation to T cells (immunosuppressants)[1][3][4] Enhancement of antigen presentation (vaccine adjuvants) Blocking T cell costimulation (e.g., CTLA-4 modulation of MHC-II/CD80/86)