B-1 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."