Contrasting effects of interleukin 4 (IL-4) on the in vitro proliferation of human B cells is due to the presence of B cell subsets in different activation states.

Our data indicate that resting B cells when activated in vitro with immobilized anti-IgM antibodies proliferate primarily with IL-4 but not with IL-2. In contrast, about 15 hours after activation the cells become responsibe to IL-2. At that stage, IL-4 becomes an inhibitory factor