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 s
IgM,
CD21,
CD1,
CD9 with low to negligible levels of s
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.