Medicine & USMLE

T Cells - Overview

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)


T-cells are lymphocytes which are part of the adaptive immune system. T-cells mediate cellular immunity, usually through direct contact with other cells. When T-cells overreact to self-antigens or other harmless antigens, undesired T-cell responses cause Type 4 hypersensitivity reactions. 

Key Points

  • T cells
    • Lymphocytes of the adaptive immune system
      • Must be activated against a specific antigen
      • Most (80%) circulating lymphocytes are T-cells 
    • Types include Helper CD4+ T-cells and Killer CD8+ T-cells
    • Mediate cell-mediated immune response 
      • Achieves its effects by direct cell-to-cell contact
    • Undesired T cell activity results in Type 4 Hypersensitivity Reactions
      • Also called delayed or cell-mediated hypersensitivity