has plasma membrane part B-1 B cell B-1a B cell CD5-positive B1 cell A B-1 B cell that has the phenotype CD5-positive. CD5(+) B1 cell B1a B lymphocyte B1a B cell B1a B-cell https://cellxgene.cziscience.com/cellguide/CL_0000820 B1a cell B-1a B lymphocyte CD5+ B1 cell B-1a B-cell B1a B-lymphocyte B-1a B cells are a subset of B cells, which are major components of the adaptive immune system, primarily responsible for humoral immunity. B-1 cells can be subdivided into B-1a and B-1b cells. Unlike their conventional B-2 B cell counterparts, B-1a B cells are self-renewing and mainly found in the peritoneal and pleural cavities, although they can also be detected in the spleen and the bone marrow. The distinguishing factor for B-1a B cells is the expression of CD5 on their cell surface, in addition to the common B cell markers, such as CD19 and CD20. In terms of function, B-1a B cells are crucial for the production of low-affinity, polyreactive natural antibodies, which are predominantly of the IgM class. These natural antibodies are produced even in the absence of pathogenic stimulation and play a substantial role in maintaining immune homeostasis. They can recognize highly conserved structures primarily found on the surface of common pathogens, and self-antigens released from damaged or dying cells. It appears that these autoreactive antibodies can contribute to autoimmunity. B-1a B cells also have a significant role in the early immune response as they can respond rapidly to pathogenic assault without requiring additional signals for activation and proliferation. This rapid response feature is mediated by the secretion of large amounts of immunoglobulins in response to Toll-like receptor (TLR) ligands. The B-1a B cells also exhibit potent phagocytic capabilities, perform antigen presentation to T cells, and exert regulatory functions through their production of IL-10, a cytokine suppressing inflammatory and autoimmune reactions. (This extended description was generated by ChatGPT and reviewed by the CellGuide team, who added references, and by the CL editors, who approved it for inclusion in CL. It may contain information that applies only to some subtypes and species, and so should not be considered definitional.) B-1a B-lymphocyte T-cell surface glycoprotein CD5