B lymphocytes (B cells) are an integral part of the humoral immune response due to their ability to produce antibodies against foreign antigens. B cells originate from hematopoietic stem cells (HSCs) in the bone marrow, which is seeded during embryonic development by HSCs from the fetal liver. The initial stages of B cell development are antigen-independent and involve the generation of several intermediary precursor cells that arise from B lymphocyte progenitor cells including Pre-pro-B cells, Pro-B cells, and Pre-B cells, which develop into immature B cells. During these stages of development, B cells undergo immunoglobulin gene rearrangement resulting in the expression of a mature B cell receptor (BCR) that is capable of binding to antigen. This is followed by a selection process that involves BCR editing or clonal deletion, designed to eliminate self-reactive immature B cells. The majority of immature B cells that survive this selection process leave the bone marrow and migrate to the spleen where they differentiate into transitional immature B cells that then become immunocompetent naïve mature B cells. Most naïve B cells develop into follicular B cells, while a small population becomes marginal zone B cells (frequently called IgM memory B cells in human). Following antigen-dependent activation, follicular B cells participate in germinal center reactions where they differentiate into memory B cells or long-lived, antibody-secreting plasma cells. While these developmental stages are similar for human B cells and mouse conventional B2 cells, a second mouse cell lineage (B1 cells) has also been described. B1 cells are abundant in the peritoneal cavities and can be further subdivided into B1a and B1b subsets. These along with other B cell precursor cells and functionally distinct subtypes including marginal zone B cells, follicular B cells, memory B cells, plasma cells, and regulatory B cells can be distinguished from each other based on the expression of specific cell surface and intracellular markers. R&D Systems and Novus Biologicals together offer the widest selection of fluorochrome-conjugated antibodies for detecting human and mouse B cell subsets and further characterizing these cells.