The marginal zone (MZ) and B1 subsets of B cells, which differ from conventional follicular (FO) B cells both developmentally and functionally, are involved in early responses to infectious pathogens and the production of self-reactive antibodies. A novel gene, mzb1, is expressed at high levels in MZ and B1 B cells but at low level, if at all, in FO B cells. MZB1 is involved in the regulation of proliferation, BCR-mediated signal transduction, and antibody production in B cells. Inhibitors, activators and enhancers of MZB1 expression or activity can be used as immune modulators for research and therapeutic purposes.