B cells, or B lymphocytes, are an immune cell subtype known for its role in secreting antibodies. B cell development begins in the bone marrow when hematopoietic stem cells differentiate into common lymphoid progenitor cells. Cell surface markers to select for B cells include CD19 and CD20. Activated B cells become plasma cells and produce large amounts of antibodies. The CD138 marker can be used to identify plasma cells. B Cell Isolation kits may separate based on positive selection and negative selection or depletion.