Memory B cell
Memory B cells are a B cell sub-type that are formed within germinal centers following primary infection and are important in generating an accelerated and more robust antibody-mediated immune response in the case of re-infection (also known as a secondary immune response).
During an initial infection (or primary immune response) involving a T-dependent antigen, naive follicular B cells are activated in the presence of TFH cells within the follicles of secondary lymphoid organs (i.e. spleen and lymph nodes) and undergo clonal expansion to produce a foci of B cells that are specific for the antigen. Most of these clones differentiate into the plasma cells, also called effector B cells which produce a first wave of protective antibodies and help clear the infection, but a fraction persist as dormant memory cells that survive in the body on a long-term basis after having gone through a highly mutative and selective germinal center reaction. Activated B cells that fail to undergo germinal center differentiation do not persist as effective memory B cells and are rapidly negatively selected against.
Within germinal centers, B cells proliferate and mutate the genetic region coding for their surface antibody (also known as immunoglobulin). The process is called somatic hypermutation and is responsible for introducing spontaneous mutations with a frequency of about 1 in every 1600 cell division (a relatively high frequency considering the low mutation frequency of other cells of the body being 1 in 106 cell divisions). Then after gaining a set number of mutations, germinal center B cells are subjected to a round of selection by TFH cells. B cell clones that have mutated and gained higher affinity surface immunoglobulin that better recognize antigen receive cellular contact-dependent survival signals from interacting with their cognate TFH cells and go on to one of three fates: (i) differentiate into plasma cells that have improved affinity towards antigen (therefore more efficient than their earlier the generation of plasma cells in clearing the infection), (ii) affinity matured memory B cells, or (iii) retained in the germinal center to re-enter another round of mutative replication and TFH cell-dependent selection. Therefore, as an infection proceeds, memory B cells selected in the later stages of a germinal center response are found to have accumulated the highest numbers of immunoglobulin mutation events with superior affinity towards their targeted antigen. Conversely, during the course of a germinal center reaction, low affinity or potentially auto-reactive germinal center B cell clones, or those that have gained non-functional mutations are out-competed by higher affinity clones and eventually undergo cellular apoptosis.
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