MHCs
Summary
MHCs (major histocompatibility complexes) are cell surface glycoproteins that present antigens to T cells and mediate immune recognition. HLA (human leukocyte antigen) is the human MHC system; Class I (present to CD8+ T cells) and Class II (present to CD4+ T cells).
Detail
The major histocompatibility complex (MHC) consists of glycoproteins that display antigenic peptides on cell surfaces for T cell recognition. The human MHC system is called human leukocyte antigen (HLA) and is encoded on chromosome 6. MHC Class I molecules (HLA-A, B, C) are expressed on all nucleated cells and present intracellular antigens (e.g., viral peptides) to CD8+ T cells (cytotoxic T lymphocytes), triggering cytotoxic responses. MHC Class II molecules (HLA-DR, DQ, DP) are expressed on antigen-presenting cells (dendritic cells, macrophages, B cells) and present extracellular antigens to CD4+ T cells (helper T cells), triggering immune activation. MHC Class I consists of a heavy chain + beta-2 microglobulin; Class II consists of alpha and beta chains. The peptide-binding groove is variable, encoded by different HLA alleles. MHC polymorphism explains transplant rejection and pathogen diversity. HLA associations are critical on boards: HLA-B27 (ankylosing spondylitis, reactive arthritis), HLA-DR3/DR4 (type 1 diabetes), HLA-DR5 (primary biliary cirrhosis). Gene frequency and allele combinations determine disease susceptibility.
Sources
- First Aid for USMLE Step 1
- Immunobiology (Janeway)
- Robbins Pathology
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