CD40 is a type I cell surface protein belonging to the tumor necrosis factor superfamily of cell surface receptors. In mice it is expressed on B lineage cells, follicular dendritic cells, thymic epithelium, and interdigitating cells in the T-cell zone of secondary lymphoid organs. CD40 first becomes detectable on a subset of small pre-B II cells in bone marrow with the levels of CD40 expression increasing thereafter during B cell maturation. Immature B cells (IgM+IgDloB220lo) express intermediate levels of CD40, whereas mature B cells (IgM+IgDhiB220hi) express high levels. CD40 has a central role in B cell growth and differentiation, and signaling through CD40 in combination with IL-4 reportedly induces immunoglobulin isotype switching and secretion of IgE. The agonistic 1C10 antibody closely resembles gp39/CD40 ligand in its ability to stimulate proliferation of small, resting B lymphocytes in the absence of other cofactors.