Although we have no direct evidence that the Ag-responsive T cells detected ex vivo include functionally active effectors, it is worthwhile mentioning that adoptive transfer of CD4+/CD8+ T cells (isolated from wild-type mice at 60 d p.i.)into B2R−/− mice rendered these recipient mice resistant to lethal infection (0% mortality, n = 5; three independent experiments), as compared to non-manipulated B2R−/− mice (100% mortality) or B2R−/− mice that received CD4+/CD8+ T cells from normal wild-type mice (100% mortality). Here, CD4 is linked to infection.