Medicine & USMLE


B Cells and T Cells
  1. MHC I
  2. MHC II
  3. B Cells - Overview
  4. B Cells - Activation
  5. Plasma B Cells
  6. Memory B Cells
  7. T Cells - Overview
  8. T Cells - Activation
  9. T Cells - Differentation
  10. Cytotoxic (Killer) T Cells
  11. Helper T Cells - Overview
  12. Th1 Cells
  13. Th2 Cells
  14. Th17 Cells
  15. Regulatory T Cells (Tregs)


Major Histocompatibility Complex I (MHC I) is a cell-surface protein found on all nucleated cells. MHC I presents endogenous antigens derived from within the cell. Antigens bound to MHC I are recognized by CD8+ Cytotoxic T-cells. When the antigen presented on MHC I is abnormal, such as in the context of viral infection or cancer, the T-cell will kill the cell presenting the abnormal antigen. Structurally, MHC I is made up of a long heavy chain and a separate short chain also known as beta-2-microglobulin. MHC I is primarily encoded by the HLA -A,-B,and -C genes. 

Key Points

  • Major Histocompatibility Complex I (MHC I)
    • Expressed on all nucleated cells, APCs, platelets (not RBCs)
    • Presents endogenous/intracellular antigens
      • Usually viral or cytosolic proteins from within the cell
      • Antigen loaded in RER via TAP (transporter associated with antigen processing)
    • Recognized by CD8+ Cytotoxic T-cells 
      • Via T cell receptors (TCRs)
    • Encoded by HLA-A, -B, -C genes
    • Composed of 1 heavy chain and 1 short chain (beta-2 microglobulin)