T Cells - Activation

Summary

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
    • On MHC I to CD8+ Killer T-cells
    • On MHC II to CD4+ Helper T-cells
  • 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