Question

Microbiology:

How many types of B cells are produced in the process of clonal expansion? Summarize why each is needed for a thorough and long-lasting humoral immune response to an antigen.

Answer 1

• Whenever our immune system encounters an antigen, it responds to the antigen by formation of B and T lymphocytes.
• By a process of allelic exclusion, each B cell is programmed to form only one class of Ig, with either the kappa or lambda chain, possessing specificity to a single epitope.
• On contacting the appropriate antigen, the mature B cells undergo clonal proliferation. Some activated B cells become long-lived memory cells, responsible for the recall phenomenon, when the same antigen is encountered by the body again. Majority of B cells are transformed into plasma cells.
• The explosive increase in the number of lymphocytes from a few to a large number, after they encounter the antigen, is called as clonal expansion.
• Plasma cells :

Plasma cells are the antibody secreting cells. Antibodies are the effector products of humoral immunity. Plasma cells are designed to be an immunoglobulin producing factory. They are end cells, and have a short life span of about 2-3 days.

A plasma cell will make an antibody, which is of a single specificity, of a single immunoglobulin class and allotype and of a single light chain only. An exception maybe seen in the primary antibody response, where a plasma cell which produces Igm initially, may later be switched to IgG production. Hence, plasma cells are important in antibody production, and these antibodies help neutralise or eliminate the antigens.

• Memory cells :

​After B cells are selected in the germinal center for those having high affinity membrane Ig for antigen, some B cells differentiate into plasma cells, and some differentiate into memory B cells. These cells express all isotypes, IgM, IgD, IgG, IgA and IgE, as compared to naive B cells, which express only IgM and IgD. they are long lived and more readily stimulated than naive cells and mediate secondary immune response to subsequent encounters with the same antigens. Hence, these cells are important in the memory of the immune system, and these cells can respond much more quickly and effectively or re-encountering the same antigen.