B Cells Immunophenotyping

B Cells Subtypes

B cell development begins in the bone marrow, where hematopoietic stem cells differentiate into lymphoid progenitor cells. The B-cell development occurs in steps, that are tightly controlled by the expression and function of the B-cell receptor (BCR). In the bone marrow, the B-lineage includes phenotypically distinct cell types in their different developmental stages. B cells are classified as transitional, mature-naïve, memory, atypical memory, activated B cells and plasmablasts according to their maturation stage and function. By flow-cytometry the different subpopulations can be recognized by the regulated expression of different combinations of cluster of differentiation (CD) markers on their surface.

Fig 1. Human B cell ontogeny
(Source: Front. Immunol. 2019.)

Transitional B Cells: B cells emigrate from the bone marrow as transitional B cells when they express a functional BCR, composed of membrane-bound antibody, capable of recognizing the antigen, associated to the B-cell signaling module represented by the Igα-Igβ heterodimer.

Mature- Naïve B Cells: Transitional B cells are short-lived and rapidly differentiate into mature-naïve B cells [8], that represent a major population in the peripheral blood and populate the primary lymphoid follicles in lymph nodes and spleen. Mature-naïve B cells continuously recirculate with the lymph and blood scrutinizing the environment in search of antigen.

Memory B Cells: Activated mature-naïve B cells proliferate, introduce mutation in their immunoglobulin genes and are selected for their affinity to the antigen. Only high-affinity B cells become either memory B cells.

Plasmablasts B Cells: Plasmablasts are differentiated B cells that provide protective immunity thanks to the continuous secretion of antibodies. Plasmablasts are precursor cells of short- and long-lived plasma cells which are the terminally differentiated elements of the B lineage. Normally, plasma cells are not found in the circulation; all antibody-secreting cells in the blood en route to, for example, the bone marrow, are plasmablasts and are still considered as a proliferating fraction of antibody-secreting cells.

Atypical Memory B Cells: Atypical memory B cells represent approximately 5% of B cells in the peripheral blood of healthy individuals. Atypical memory B cells have been described in aged mice and humans, increase during autoimmune diseases and in viral infections and are thought to reflect a failure or impairment of the germinal center reaction.

Activated Memory B Cells: These activated memory B cells have been found to be different from plasmablasts from the transcriptional point of view and still committed to the memory lineage. Activated memory B cells are identified as CD27+ and CD21-.