Cytotoxic T lymphocyte

Effector CD8 T cells (CTLs) recognize peptide bound to MHC class I.

Activation
Requires 3 signals: In some cases, a CD4 T cell being activated by the same DC can provide additional IL-2.
 * 1) TCR : MHC I-peptide + co-receptor engagement
 * 2) CD28 : B7.1/.2 signalling (co-stimulation)
 * 3) Requirement for additional IL-2

Differentiation
The differentiation process involves changes in the expression of adhesion molecules to allow entry to infection sites. Effector CD8 T cells express high levels of CD2 and LFA-1.

Killing
When an effector CD8 T cell recognizes MHC I-peptide, LFA-1 undergoes a conformational change to bind more tightly to ICAM-1 on the infected cell. Effector CD8 T cells do not need co-stimulation to function.

The CTL releases granules to direct the infected cell to undergo apoptosis rather than cell lysis. This ensures that only the target cell is killed, and not healthy bystander cells or the CTL itself, and that no new virus particles are released as the target cell is being killed.

Infected cells that have undergone apoptosis are then phagocytosed by macrophages. CTLs also secrete IFN-γ to stimulate nearby macrophages so the latter can phagocytose more effectively.