Co-regulation of PD-1 acquisition and functional avidity of T cells during immune priming. A and B show schematically the key parameters controlling two complementary features of T cells resulting from immune priming: PD-1 expression (A) and the functional avidity (B). Effective priming warrants optimal, balanced exposure to TCR-dependent and independent stimuli ("green zone") resulting in T cells with a desired effector profile upon boosting. Please note the inverse relationship between functional avidity and the amount of antigen. The table (bottom) depicts the major, synergistic features of priming and boosting vectors/regimens, as a pre-requisite to designing superior vaccination strategies. The model is based on published research (eg. refs [40, 42, 48, 59, 60]).