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w 5 heavy AAA^^ chain why a B cell expresses both IgM and IgD rather than IgM alone. The coexpression of IgM and IgD is accompanied by the ability to recirculate and the acquisition of functional competence, and this is why IgM+IgD+ B cells are also called mature B cells. This correlation between expression of IgD and acquisition of functional competence has led to the suggestion that IgD is the essential activating receptor of mature B cells. However, there is no evidence for a functional difference between membrane IgM and membrane IgD. Moreover, knockout of the Ig 8 gene in mice does not have a significant impact on the maturation or antigen-induced responses of B cells. Follicular B cells are also often called recirculating B cells because they migrate from one lymphoid organ to the next, residing in specialized niches known as B cell follicles (see Chapter 2). In these niches, these B cells are maintained, in part, by survival signals delivered by a cytokine of the tumor necrosis factor (TNF) family called BAFF or BlyS (see Chapter 11).

Mature, naive B cells are responsive to antigens, and unless the cells encounter antigens that they recognize with high affinity and respond to, they die in a few months. In Chapter 11, we will discuss how these cells respond to antigens and how the pattern of Ig gene expression changes during antigen-induced B cell differentiation.

B-1 and Marginal Zone B Cells

A subset of B lymphocytes, called B-1 B cells, differs from the majority of B lymphocytes and develops in a unique manner. These cells develop from fetal liver-derived HSCs and are best defined in rodents. Most murine B-1 cells express the CD5 (Ly-1) molecule. In the adult, large numbers of B-1 cells are found as a self-renewing population in the peritoneum and mucosal sites. B-1 cells develop earlier during ontogeny than conventional B cells do, express a relatively limited repertoire of V genes, and exhibit far less junctional diversity than conventional B cells do (because TdT is not expressed in the fetal liver). B-1 cells as well as marginal zone B cells spontaneously secrete IgM antibodies that often react with microbial polysaccharides and lipids. These antibodies are sometimes called natural antibodies because they are present in individuals without overt immunization, although it is possible that microbial flora in the gut are the source of antigens that stimulate their production. B-1 cells contribute to rapid antibody production against microbes in particular tissues, such as the peritoneum. At mucosal sites, as many as half the IgA-secreting cells in the lamina propria may be derived from B-1 cells. B-1 B cells are analogous to y8 T cells in that they both have antigen receptor repertoires of limited diversity, and they are both presumed to respond to commonly encountered microbial antigens early in immune responses.

The major marker used to delineate murine B-1 cells is CD5. In humans, B-1-like cells have been described but these cells do not express CD5. In humans, CD5 is found on transitional B cells and some activated B cell populations.

Marginal zone B cells are located primarily in the vicinity of the marginal sinus in the spleen and are similar to B-1 cells in terms of their limited diversity and their ability to respond to polysaccharide antigens and to generate natural antibodies. Marginal zone B cells exist in both mice and humans and express IgM and the CD21 coreceptor. In mice, marginal zone B cells exist only in the spleen, whereas in humans, they can be found in the spleen as well as in lymph nodes. Marginal zone B cells respond very rapidly to blood-borne microbes and differentiate into short-lived IgM-secreting plasma cells. Although they generally mediate T cell-independent humoral immune responses to circulating pathogens, marginal zone B cells also appear capable of mediating some T cell-dependent immune responses.

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