Longitudinal CD4 responses. Blood was taken approximately 1 month apart from 2 subjects (3 blood draws in total) and a PBMC sample was frozen from each blood draw. PBMCs from all blood draws were then thawed and tested at the same time in the CD4 IFN-γ ELISPOT assay. The number of ELISPOTS/well is plotted on the Y-axis. The amount of cells placed in each well are those recovered from an original 0.5 million PBMCs.