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 sIgM, CD21, CD1, CD9 with low to negligible levels of sIgD, 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 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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