Medicine & USMLE

T Cells - Activation

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-cell Activation is a process by which antigens are presented to resting T-cells, causing them to activate and initiate an immune response.  Different signals are required depending on the type of T-cell being activated. CD4+ or Helper T-cells are activated when MHC II on the presenting cell binds their T-cell receptor. Helper T-cells also require a second costimulatory signal, in which B7 on presenting cells binds CD28 on the Helper T-cell. CD8+ or Killer T-cells are simpler, and only require 1 signal: the binding of MHC I from the presenting cell to their T-cell receptor. Once activated, both T-cell types proliferate and turn into effector subtypes to execute their respective functions.

Key Points

  • T Cell Activation
    • Begins with Antigen Presentation
    • Antigen binds to T Cell Receptor (signal 1)
      • Sufficient to activate CD8+ Killer T Cells
    • CD4+ Helper T Cells require a costimulatory signal (signal 2)
      • B7 protein (CD80/86) on APC binds CD28 on Helper T Cell
    • Activation and differentiation into effector subtypes
      • Helper T-cells differentiate into Th1, Th2, or Th17 subtypes, and coordinate immune responses against activating antigen
      • Killer T-cells activate directly kill cells presenting activating antigen