phenotype affects entity B-1 B cell abnormal B-1 B cell morphology any structural anomaly of the subset of B cells found predominantly in the peritoneum, pleural cavities, and spleen, and 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 abnormal B-1 B lymphocyte morphology abnormal B-1 B-cell morphology abnormal B1 cell morphology abnormal B-1 B-lymphocyte morphology MPheno.ontology MP:0004940 abnormal mature B cell morphology