in an antibody producing cell, a switch in the class of antibody being synthesized
Source: Jenkins, John B. 1990. Human Genetics, 2nd Edition. New York: Harper & Row ...
Class switching: The process by which a IgM or IgD producing B lymphocyte switches to produce one of the secondary immunoglobulins (IgG, IgA or IgE) with the same antigen binding specificity.
Isotype or class switching is a biological process occurring after activation of the B cell, which allows the cell to produce different classes of antibody (IgA, IgE, or IgG).
antibody variable regions; that is, to accomplish V(D)J joining — (mice whose Ku80 genes have been knocked out cannot do this);
different antibody classes; that is, to accomplish class switching.
Antibody class switching declines significantly once helper T cell function fails. The immune system loses its ability to improve the affinity of their antibodies, and are unable to generate B cells that can produce antibody groups such as IgG and IgA.
Indeed null mutations that block class switching to IgA have significant effects on microbiota composition ,.
See also: What is the meaning of Class, Human, Trans, Antibody, Protein?