Origin, functions, heterogeneity and associated diseases of B-1 cells.

B-1 cells are derived from a subpopulation of B lymphocytes which have a specific developmental process, unique phenotype and location, and distinct functions in comparison with conventional B-2 cells. The origin of B-1 cells is not completely clear, with two existing hypotheses concerning their lineage and differentiation pathways. B-1 cells are located principally in the peritoneal and pleural cavities, but are also distributed in secondary lymphoid tissues, at mucosal sites and in the blood and bone marrow. B-1 cells regulate immune responses and maintain homeostasis by secretion of natural antibodies (nAbs), and participate in the adaptive immune response through phagocytosis and presentation of antigens to T cells. B-1 cells are associated with many diseases including autoimmune, infectious and inflammatory diseases. This review focuses on the origin and biological functions of B-1 cells as well as their involvement in human disease, and discusses advances in the understanding of the heterogeneity of B-1 cells under specific pathophysiological features, as partly clarified by single-cell sequencing analysis.