IgE isotype switching of human B cells requires physical interaction of T and B cells via surface molecules, and either IL-4 or IL-13 secreted by T cells. In this study we analyzed the role of IL-4 versus IL-13 in IgE production in atopy. We found that peripheral blood mononuclear cells (PBMC) from atopic individuals but not from nonatopic subjects secreted IgE without addition of IL-4 or IL-13, if T and B cells were simultaneously activated by anti-CD3 mAb and soluble CD40L, respectively. IgE production by atopic PBMC was dependent on endogenously secreted IL-4 and IL-13, since it could be blocked by a combination of anti-IL-4 plus anti-IL-13 antibodies. No differences in the B cell compartment of nonatopics and atopics were detectable, since PBMC from both donor populations secreted comparable amounts of IgE, if only the B cells were activated by soluble CD40L plus either exogenous IL-4 or IL-13. Further phenotypic analysis of T cells from atopics revealed that activated CD4+45RO- secreted IL-4 but no IL-13, whereas CD4+45RO+ memory T cells secreted low amounts of IL-4, but large amounts of IL-13. Accordingly, prolonged activation of native CD4+45RO- T cells in vitro induced expression of CD45RO, and strongly favored secretion of IL-13 rather than IL-4. Addition of exogenous IL-4 during activation further increased both IL-4 and IL-13 production to a similar degree. However, the potential of CD4 T cells from atopics to deliver contact-dependent activation signals to B cells and to induce IgE production (in the absence of soluble CD40L) increased with prolonged activation, and coincided with IL-13 rather than IL-4 production. Under similar conditions, CD8 effector cells secreted IL-13 but no IL-4, did not express CD40L, and could not help Ig(E) production by B cells. These results suggest that, in atopy, persistently stimulated CD4+45RO+memory/effector T cells provide contact-dependent activation signals to B cells, and that these cells may induce IgE switching largely via secretion of IL-13.