CD4 and infection: In most general terms, the efficacy of a secondary (IIo) CD8+ effector T cell (CD8+TE) response is contingent on the numbers of available CD8+TM, their differentiation status and anatomical distribution, the contribution of other immune cell populations (e. g., CD4+T cells, B cells, innate immune cells), and the precise conditions of pathogen re-encounter, i. e. the nature of the pathogen as well as the route and dosage of infection.