There are two broad populations of B cells, B-1 and B-2. B-1 cells are the primary B cell during fetal and neonatal development. These cells can self-renew and localize mainly in the peritoneal and pleural cavities. B-1 cells produce the majority of “natural” antibodies, immunoglobulin (Ig) M and IgA. Natural antibodies exist in the blood of healthy individuals before immunization, are the first line of defense against pathogens (disease-causing antigens), and they also influence T cell expansion.