CD4 and viral infectious disease: Our experimental design that utilized intramuscular immunization for priming is of particular importance, as the antigen dose is limited and the dLNs for the i.m. route are remote to the mediastinal LNs, ensuring that the GP66-77 epitope-specific CD4+ T cell response upon influenza challenge is generated from memory Tfh cells and not residual effector Tfh cells in GCs that may have persisted after acute viral infection.