three major naive peripheral B-cell populations B cell MARGINAL ZONE FOLLICULAR B1 B CELLS B CELLS B CELLS IgM IgM CD21 IgM IgD CD21 CD23 Mac- CD5 CD1 T-independent responses T-dependent responses T-independent responses ?T-dependent responses T-independent responses Mucosal Immunity Immunity to blood borne Immune responses in mos B1a cells primarily of pathogens secondary lymphoid organs fetal liver origin Self-renewing? High-affinity IgG Self-renewing Immunol rev 2004: 197: 206three major naive peripheral B-cell populations B cell Immunol Rev 2004; 197:206 High-affinity IgG