Bcell

B cells, or B lymphocytes, are a type of white blood cell that plays a central role in the adaptive immune system. They develop from hematopoietic stem cells in the bone marrow, undergoing maturation before entering the circulation as naive B cells. Each B cell expresses a unique B cell receptor (BCR), a membrane-bound immunoglobulin that recognizes a specific antigen. BCR diversity arises through V(D)J recombination of immunoglobulin gene segments.

Upon antigen encounter, B cells can be activated directly or with help from CD4+ T helper cells. Activation triggers clonal expansion and differentiation. In germinal centers of lymphoid tissues, B cells undergo somatic hypermutation and class-switch recombination, increasing antibody affinity and changing antibody isotypes. Most activated B cells become antibody-secreting plasma cells, which produce antibodies that neutralize pathogens or mark them for destruction. Others become memory B cells, which persist for years and provide long-term immunity.

B cells are the main effectors of humoral immunity and exist in various subtypes, including follicular B cells, marginal zone B cells, and B1 cells in some species. They express surface markers such as CD19, CD20, CD21, and CD22, with additional markers like CD27 aiding identification of memory B cells. Regulatory interactions with BAFF signaling through the BAFF receptor support B cell survival and maturation.

Clinical relevance includes roles in immunodeficiencies, autoimmune diseases, and B cell–derived cancers (e.g., leukemia, lymphoma, myeloma). Therapies targeting B cells include anti-CD20 monoclonal antibodies (such as rituximab), other antibodies, BAFF pathway inhibitors, and chimeric antigen receptor (CAR) T-cell therapies directed at CD19. The term B cell reflects historical naming from the Bursa of Fabricius in birds, though the cells are central to mammalian immunity as well.