B-1 B cell morphology trait Any measurable or observable characteristic related to the shape, structure, color, or pattern 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. B-1 B lymphocyte morphology trait B-1 B-cell morphology trait B-1 B-lymphocyte morphology trait B1 cell morphology trait Trait.ontology VT:0004940 mature B cell morphology trait