Final answer:
Allelic exclusion in B cells ensures that each B cell expresses only one heavy and one light chain, essential for the specificity of antibody produced by B cells. This genetic mechanism leads to the production of B cells with unique antigen-binding sites, preventing the expression of multiple antibody specificities that could cause autoimmunity.
Step-by-step explanation:
Allelic exclusion ensures that B cells express only one heavy and one light chain. This process is crucial in the development of B cells and contributes to their ability to produce a unique antibody that binds to a specific antigen. During B cell maturation, genetic rearrangements take place where an enzyme called DNA recombinase excises and randomly combines V (variable), D (diversity), and J (joining) gene segments for the heavy chain, and V and J segments for the light chain. This recombination event is followed by selective mRNA splicing that results in a B cell with a unique variable region for the antibody it will produce. Once a functional heavy chain and light chain are produced, allelic exclusion ensures that the B cell suppresses rearrangement of other alleles, stabilizing the specificity of the B cell receptor.
Allelic exclusion is critical for preventing the production of B cells with multiple specificities, which could potentially result in autoimmune diseases. It also means that B cells do not undergo isotype switching at this stage and do not proliferate without stimulation. This specificity is established before encountering an antigen, and upon stimulation by an antigen, a B cell can then proliferate and differentiate further, such as undergoing isotype switching or differentiating into plasma cells to secrete antibodies.