Major histocompatibility complexes
Summary
Major histocompatibility complexes (MHCs) are cell surface molecules presenting antigens to T cells. MHC Class I (all cells, CD8+ T cells) and Class II (APCs, CD4+ T cells); essential for adaptive immunity and transplant rejection.
Detail
Major histocompatibility complexes (MHCs) are a family of cell surface glycoproteins central to adaptive immunity. MHC Class I molecules present intracellular-derived peptides to CD8+ cytotoxic T lymphocytes. They are found on all nucleated cells and consist of a heavy chain (45 kDa) non-covalently bound to beta-2 microglobulin (12 kDa). The peptide-binding groove accommodates 8-10 amino acid peptides. MHC Class II molecules present extracellular-derived peptides to CD4+ helper T cells. They are expressed on professional antigen-presenting cells (dendritic cells, macrophages, B cells) and consist of alpha and beta chains with a peptide-binding groove accommodating 13-25 amino acid peptides. Human MHC genes (HLA genes) are the most polymorphic in the human genome, encoded on chromosome 6. Class I genes include HLA-A, HLA-B, and HLA-C. Class II genes include HLA-DR, HLA-DQ, and HLA-DP. This polymorphism ensures population-level recognition of diverse pathogens but complicates transplantation (allogeneic transplant rejection occurs due to MHC mismatch). MHC-peptide-TCR interaction is the critical signal for T cell activation. Understanding MHC is fundamental to immunology, transplantation medicine, and disease associations.
Sources
- First Aid for USMLE Step 1
- Immunobiology (Janeway)
- Harrison's Principles of Internal Medicine
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