Marginal zone B-cell
Marginal zone B cells are noncirculating mature B cells that segregate anatomically into the marginal zone (MZ) of the spleen. This region contains multiple subtypes of macrophages, dendritic cells, and the MZ B cells; it is not fully formed until 2 to 3 weeks after birth in rodents and 1 to 2 years in humans. The MZ B cells within this region typically express high levels of IgM, CD21, CD1, CD9 with low to negligible levels of secreted-IgD, CD23, CD5, and CD11b that help to distinguish them phenotypically from follicular (FO) B cells and B1 B cells. In humans the splenic marginal zone B cells have evidence of somatic hypermutation in their immunoglobulin genes, indicating that they have been generated through a germinal centre reaction to become memory cells. 
Similar to B1 B cells, MZ B cells can be rapidly recruited into the early adaptive immune responses in a T cell independent manner. The MZ B cells are especially well positioned as a first line of defense against systemic blood-borne antigens that enter the circulation and become trapped in the spleen. It is believed they are especially reactive to bacterial cell wall components and self-antigens which are the products of aging. MZ B cells also display a lower activation threshold than their FO B cell counterparts with heightened propensity for plasma cell differentiation that contributes further to the accelerated primary antibody response.
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