Follicular B cells (FO B cells) are a type of B cell that reside in primary and secondary lymphoid follicles (containing germinal centers) of secondary and tertiary lymphoid organs, including spleen and lymph nodes.
Mature B cells from the spleen can be divided into two main populations: FO B cells, which constitute the majority, and marginal zone B-cells, lining outside the marginal sinus and bordering the red pulp. FO B cells express high levels of IgM, IgD, and CD23; lower levels of CD21; and no CD1 or CD5, readily distinguishing this compartment from B1 B cells and marginal zone B-cells . FO B cells organize into the primary follicles of B cell zones focused around follicular dendritic cells in the white pulp of the spleen and the cortical areas of peripheral lymph nodes. Multiphoton-based live imaging of lymph nodes indicate continuous movement of FO B cells within these follicular areas at velocities of ~6 µm per min. Recent studies indicate movement along the processes of FDC as a guidance system for mature resting B cells in peripheral lymph nodes. Unlike their MZ counterpart, FO B cells freely recirculate, comprising >95% of the B cells in peripheral lymph nodes.
The BCR repertoire of the follicular B cell compartment also appears under positive selection pressures during final maturation in the spleen. However, diversity is substantially broader than B1 B and MZ B cell compartments. More importantly, FO B cells require CD40-CD40L dependent TFH cell help to promote effective primary immune responses and antibody isotype switching and to establish high-affinity B cell memory.