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The terminal differentiation of B cells into antibody-secreting cells (ASCs) is the basis of hu-moral immunity. After birth, B cell development begins in the BM from where selected immature B cells migrate to the spleen. There, immature B cells progress into T2 B cells and subsequently into the B2 B cell lineage, namely into marginal zone (MZ) B cells, or follicular (FO) B cells that recirculate through the lymphoid follicles of spleen and lymph nodes (Loder et al., 1999). Another B cell subtype, called B1 B cells, is found predominantly in the pleural and intra-peritoneal cavities either as B1a B cells (CD11b, CD5 double positive) or B1b B cells (CD11b positive, CD5 negative; Martin et al., 2001). Upon activation, B cells divide several times and can differentiate into plasmablasts, plasma cells, or memory B cells (Manz et al., 2005). Depending on the activating signal, distinct B cell subsets preferentially contribute to the hu-moral immune response. MZ and B1 B cells have the unique capacity to quickly respond to spe-cific bacterial side products like LPS, and differ-entiate into plasmablasts and short-lived plasma cells producing large amounts of IgM as well as isotype-switched antibodies (Lopes-Carvalh