A B cell is a type of white blood cell belonging to the lymphocyte group within the immune system. B cells are central to adaptive immunity, particularly its humoral branch, which operates through antibodies in blood and other body fluids. They recognize molecular targets called antigens through receptors on their surface. After appropriate activation, they can multiply and differentiate into antibody-secreting plasma cells or memory B cells. They also participate in antigen presentation and immune regulation. (immunology.org)
Name and development
The designation “B” derives from the bursa of Fabricius, an organ in birds where B cells develop. Experiments in chickens during the 1960s helped establish the distinction between antibody-producing lymphocytes and other lymphocyte populations. Although mammalian B cells develop principally in bone marrow, their name did not originally abbreviate “bone marrow.” (immunology.org)
In mammals, B-cell development begins during fetal life, including in the fetal liver, and subsequently occurs in bone marrow. Developing cells pass through progenitor, pro-B, pre-B, and immature B-cell stages. These stages are distinguished by changes in immunoglobulin assembly and receptor expression. Immature cells leave the marrow and undergo additional maturation in peripheral lymphoid tissues, especially the spleen. Mature, antigen-inexperienced cells are termed naive B cells. (ncbi.nlm.nih.gov)
Antigen receptors and diversity
The B-cell receptor (BCR) combines a membrane-bound immunoglobulin with associated signaling components. The immunoglobulin binds antigen, while the associated components transmit signals into the cell. Developing B cells first assemble an immunoglobulin heavy chain, which is tested in a pre-B-cell receptor, and subsequently assemble a light chain. Immature B cells display IgM; mature naive cells generally express both IgM and IgD. (immunology.org)
Receptor diversity arises through V(D)J recombination, a rearrangement of gene segments in developing lymphocytes. Heavy-chain variable regions use variable, diversity, and joining segments; light-chain variable regions use variable and joining segments. Different segment combinations, changes at their junctions, and pairing of heavy and light chains generate a large repertoire of antigen-binding structures. This diversity develops before exposure to a particular foreign antigen rather than being designed in response to it. (immunology.org)
Unlike conventional T cells, which recognize processed antigen in association with presentation molecules, B cells can bind intact antigens directly. Antibody binding may block a pathogen’s entry into host cells or facilitate its destruction through phagocytosis and the complement system. (immunology.org)
Activation and cooperation with T cells
B-cell activation usually occurs in secondary lymphoid tissues, including lymph nodes and the spleen. Antigen binding initiates receptor signaling, but the resulting response depends on additional signals and the cell’s developmental state. Activated cells undergo clonal expansion, increasing the number of cells capable of responding to the recognized antigen. (ncbi.nlm.nih.gov)
Responses to most protein antigens require helper T-cell participation. A B cell takes up bound antigen, processes it, and acts as an antigen-presenting cell, displaying peptides through class II major histocompatibility complex molecules. A compatible helper T cell provides signals, including interactions involving CD40 and its ligand and secreted cytokines, that support B-cell proliferation and differentiation. (ncbi.nlm.nih.gov)
Some antigens, notably certain repetitive microbial polysaccharides, can induce responses without conventional helper T-cell assistance. Such T-independent responses differ from typical T-dependent responses in their capacity for antibody diversification and durable memory. (ncbi.nlm.nih.gov)
Germinal centers and antibody maturation
Activated B cells may produce an early response outside follicles, generating antibody-secreting plasmablasts and short-lived plasma cells. Other activated cells enter a germinal center, a specialized structure in lymphoid follicles where proliferation, antibody diversification, and selection occur. (immunology.org)
During somatic hypermutation, changes accumulate in immunoglobulin variable-region genes. Selection favors cells whose receptors bind the available antigen effectively, producing affinity maturation. Mutation alone does not guarantee improved binding; the selection process is essential. (immunology.org)
Class-switch recombination changes the immunoglobulin heavy-chain constant region, allowing production of classes such as IgG, IgA, or IgE instead of IgM. This alters the antibody’s effector functions without itself replacing its antigen-binding variable region. Class switching and affinity maturation are therefore distinct processes. (ncbi.nlm.nih.gov)
Plasma cells specialize in antibody secretion, whereas memory B cells retain the capacity to respond following renewed antigen exposure. Long-lived plasma cells can maintain antibody production, while memory B cells contribute to recall responses. These complementary populations support immunological memory. (pmc.ncbi.nlm.nih.gov)
Self-tolerance and biological significance
Because receptor generation can produce self-reactive cells, B-cell development includes central tolerance checkpoints. Receptor editing can replace a self-reactive receptor through further gene rearrangement. Other outcomes include clonal deletion and anergy, a state of reduced responsiveness. Peripheral tolerance provides additional controls after cells leave bone marrow. (ncbi.nlm.nih.gov)
Failure of these controls can contribute to autoimmunity and the production of self-reactive antibodies. B-lineage cells can also undergo malignant transformation: some forms of lymphoma arise from B cells, while multiple myeloma arises from plasma cells. These diseases involve distinct developmental stages and biological properties rather than a single uniform B-cell disorder. (ncbi.nlm.nih.gov)