B-1 B cell morphology trait B-1 B cell quantity B-1 B cell number B1 cell number The proportion or number of the subset of B cells found predominantly in the peritoneum, pleural cavities, and spleen, which are enriched for self-reactivity; 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. Trait.ontology VT:0004976 mature B cell quantity