mature B cell B-1 B cell A B cell of distinct lineage and surface marker expression. B-1 B cells are thought to be the primary source of natural IgM immunoglobulin, that is, IgM produced in large quantities without prior antigenic stimulation and generally reactive against various microorganisms, as well as the source of T-independent IgA immunoglobulin in the mucosal areas. These cells are CD43-positive. B-1 B lymphocyte B-1 B-cell B-1 B-lymphocyte B-1 cell B1 B cell B1 B lymphocyte B1 B-cell B1 B-lymphocyte B1 cell There are small numbers of B-1 cells found in the lymph nodes and spleen, while larger numbers can be found in the peritoneal and pleural cavities. B-1 B cells are reportedly CD11b-positive, CD20-positive, CD21-positive, CD27-positive, CD44-positive, CD45RB-positive, CD48-positive, CD70-negative, CD150-positive, CD244-negative, CD352-positive, sIgM-positive, and sIgD-low. cell