B-1 cells comprise a small percentage of the B lymphocytes that reside in multiple tissues including theperitoneal and pleural cavities. Functionally, B-1 cells participate in innate immunity by producing the majority of the natural IgM in serum, which protects against invading pathogens before the onset of the adaptive immune response. B-1 cells arise from fetal and neonatal progenitors and are distinct from the adult bone marrow progenitors that give rise to follicular and marginal zone B-2 cells. Recent studies have attempted to delineate the progenitors of B-1 cells from those of B-2 cells. Experimental  data from studies supporting the hypothesis that B-1 cells belong to a developmental lineage distinct from B-2 cells, and draws attention to recent studies that have defined new roles for the B-1 a and B-1 b cell subsets in the response to bacteria and self-antigens.