An antibody (Ab) is the secreted form of the B cell receptor; the term immunoglobulin refers to either the membrane-bound form or the secreted form of the B cell receptor, but they are, broadly speaking, the same protein, and so the terms are often treated as synonymous. Antibodies belong to the immunoglobulin superfamily. They are large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses, including those that cause disease. Antibodies, however, can be generated to recognize virtually any molecule in existence. The thing recognized by an antibody called an antigen. This term literally means "antibody generator." It is the presence of an antigen that drives the formation of antigen-specific antibody. Each tip of the "Y" of an antibody contains a paratope that is specific for one particular epitope on an antigen, allowing these two structures to bind together with precision. Using this binding mechanism, an antibody can tag a microbe or an infected cell for attack by other parts of the immune system, or can neutralize it directly (for example, by blocking a part of a virus that is essential for its invasion). To allow the immune system to recognize millions of different antigens, the antigen-binding sites at both tips of the antibody come in an equally wide variety. In contrast, the remainder of the antibody is relatively constant. In humans, antibodies occur in 5 classes, sometimes called isotypes: IgA, IgD, IgE, IgG, and IgM. IgG and IgA in humans are also divided into discrete subclasses (IgG1, IgG2, IgG3, IgG4; IgA1 and IgA2). The class determines the functions triggered by an antibody (also known as effector functions), in addition to some other structural features. Antibodies from different classes also differ in where they are released in the body and at what stage of an immune response. Importantly, while classes and subclasses of antibody may be shared between species (at least in name), their functions and distribution throughout the body may be different. For example, mouse IgG1 is closer to human IgG2 than human IgG1 in terms of its function. The term humoral immunity is often treated as being synonymous with the antibody response, describing the function of the immune system that exists in the body's humors (fluids) in the form of soluble proteins, as distinct from cell-mediated immunity, which generally describes the responses of T cells (especially cytotoxic T cells). In general, antibodies are considered part of the adaptive immune system. However, this classification can become blurry. For example, natural IgM, which are made by B-1 lineage cells that have properties more similar to innate immune cells than adaptive, refers to IgM antibodies made independently of an immune response that demonstrate polyreactivity- they recognize multiple distinct (unrelated) antigens. These can work with the complement system in the earliest phases of an immune response to help facilitate clearance of the offending antigen and delivery of the resulting immune complexes to the lymph nodes or spleen for initiation of an immune response. In the course of an immune response, B cells can progressively differentiate into antibody-secreting cells (B cells themselves do not secrete antibody; B cells do however express B cell receptors, the membrane-bound form of the antibody, on their surface) or memory B cells. Antibody-secreting cells comprise plasmablasts and plasma cells, which differ in mainly in the degree to which they secrete antibody, their lifespan, metabolic adaptations, and surface markers. Plasmablasts are rapidly proliferating, short-lived cells produced in the early phases of the immune response (classically described as arising extrafollicularly rather than from the germinal center) which have the potential to differentiate further into plasma cells. It should be noted however that the literature is sloppy at times and often describes plasmablasts as just short-lived plasma cells- formally this is incorrect. Plasma cells, in contrast, do not divide (they are terminally differentiated), and rely on survival niches comprising specific cell types and cytokines to persist. Plasma cells will secrete huge quantities of antibody regardless of whether or not their cognate antigen is present, ensuring that antibody levels to the antigen in question do not fall to 0, provided the plasma cell stays alive. The rate of antibody secretion, however, can be regulated, for example, by the presence of adjuvant molecules that stimulate the immune response such as TLR ligands. Long-lived plasma cells can live for potentially the entire lifetime of the organism. Classically, the survival niches that house long-lived plasma cells reside in the bone marrow, though it cannot be assumed that any given plasma cell in the bone marrow will be long-lived. However, other work indicates that survival niches can readily be established within the mucosal tissues- though the classes of antibodies involved show a different hierarchy from those in the bone marrow. B cells can also differentiate into memory B cells which can persist for decades similarly to long-lived plasma cells. These cells can be rapidly recalled in a secondary immune response, undergoing class switching, affinity maturation, and differentiating into antibody-secreting cells. Antibodies are central to the immune protection elicited by most vaccines and infections (although other components of the immune system certainly participate and for some diseases are considerably more important than antibodies e.g. herpes zoster). Durable protection from infections, that is, the ability of the microbe to enter the body and begin to replicate, (but not necessarily disease) caused by a given microbe depends on sustained production of high level of antibodies, meaning that effective vaccines ideally elicit persistent high levels of antibody, which relies on long-lived plasma cells. At the same time, many microbes of medical importance have the ability to mutate to escape antibodies elicited by prior infections, and long-lived plasma cells cannot undergo affinity maturation or class switching. This is compensated for through memory B cells: novel variants of a microbe that still retain structural features of previously encountered antigens can elicit memory B cell responses that adapt to those changes. It has been suggested that long-lived plasma cells secrete B cell receptors with higher affinity than those on the surfaces of memory B cells, but findings are not entirely consistent on this point. Source: Wikipedia (en)