T-cell response to phytohemagglutinin (PHA) and tetanus toxoid before and after vaccination. (A) PBMCs were collected at the indicated time points and incubated with 2 μg/mL of PHA for 3 days. Proliferation was measured by incorporation of tritiated thymidine. Values are presented as mean stimulation index (SI; proliferation of stimulated/unstimulated cells ± SE). (B) PBMCs were collected at the indicated time points and incubated with tetanus toxoid at 10 μg/mL for 5 days. Proliferation was measured by incorporation of tritiated thymidine. Values are presented as mean stimulation index (proliferation of stimulated/unstimulated cells ± SE).