Memory B cell
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Primary response, paratopes, and epitopes
In wake of first (primary response) infection involving a particular antigen, the responding naïve cells (ones which have never been exposed to the antigen) undergo clonal selection to produce a colony of cells that are all specific for the antigen. Most of these clones differentiate into the plasma cells, also called effector B cells (which produce the antibodies) and clear away with the resolution of infection, and the rest persist as memory cells which survive for an average of ten years.
To understand the events taking place, it is important to appreciate that the antibody molecules present on a clone (a group of genetically identical cells) of B cells have a unique paratope (the sequence of amino acids that binds to the epitope on an antigen). This allows them to bind only the epitope they are specific to.
And, each time these cells are induced to proliferate due to an infection, the genetic region coding for the paratope undergoes spontaneous mutations with a frequency of about 1 in every 1600 cell-divisions (this is a very high frequency considering the frequency with which these cells divide; compare with frequency of mutations in other cells—1 in 106). This process is known as somatic hypermutation.
Secondary response and memory
All these events occur in the highly "eventful" germinal centers of lymphoid follicles, within the lymph nodes.
Some of the resulting paratopes (and the cells elaborating them) have a better affinity for the antigen (actually, the epitope) and are more likely to proliferate than the others.
Moreover, with each such exposure to the antigen the number of different clones responding to the same antigen increases (polyclonal response), and a greater number of memory cells persist. Thus, a stronger (basically, larger number of antibody molecules) and more specific antibody-production are the hallmarks of secondary antibody response.
The facts that all the cells of a single clone elaborate one and only one paratope, and that the memory cells survive for long periods, are what impart a "memory" to the immune response.
This is the principle behind vaccination and administration of booster.
- Airoldi I, Raffaghello L, Cocco C, et al. (January 2004). "Heterogeneous expression of interleukin-18 and its receptor in B-cell lymphoproliferative disorders deriving from naive, germinal center, and memory B lymphocytes". Clin. Cancer Res. 10 (1 Pt 1): 144–54. doi:10.1158/1078-0432.CCR-1026-3. PMID 14734463.
- Lang ML (Aug 2009). "How do natural killer T cells help B cells?". Expert Rev Vaccines 8 (8): 1109–21. doi:10.1586/erv.09.56. PMC 2747240. PMID 19627191.
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