CD69 and infection: CD8+ T cells dynamically change in amounts and types at different stages of infection: (1) the expansion phase (0–7 days) with CD8+ T cells actively proliferating, (2) the peak of expansion (day 8) with effector CD8+ T cells up to the maximum, (3) the contraction phase (8–15 days) with majority of effector CD8+ T cells confronting apoptosis, and (4) the memory phase (>30 days) with only a small population of cells surviving and differentiated into various types of memory cells, namely, CD44+CD62L− TEM, CD44+CD62L+ TCM, and CD69+CD103+ TRM [7,39].