Final answer:
Allelic exclusion ensures that each B cell expresses only one unique antigen-binding specificity by expressing a single type of heavy chain and one type of light chain. This results from specific DNA rearrangement events during the maturation process, where a unique combination of VJ (light chain) or VDJ (heavy chain) segments is selected to form the variable regions of antibodies.
Step-by-step explanation:
The phenomenon of allelic exclusion ensures that each B cell expresses only one type of heavy chain and one type of light chain. During the process of B cell development, genetic rearrangement takes place. This mechanism involves DNA recombinases that cut and splice variable (V), diversity (D), and joining (J) segments of the immunoglobulin (Ig) genes. Although a B cell has the genetic potential to produce multiple forms of antibody molecules, certain recombination events are selected, leading to the production of a single unique antibody. Thus, allelic exclusion results in a B cell expressing a specific and unique antigen-binding specificity.
During B cell maturation, for instance, DNA recombinases randomly excise most of the V and J segments from the light chain gene. Only one combination of the V and J segments is retained, whereas all others are spliced out during RNA processing. We thus observe over 106 potential VJ combinations for the light chain, contributing to the immense diversity of antibodies. The constant domains, however, do not contribute to binding and remain the same across all antibodies. The variable regions formed by successful VJ (light chain) or VDJ (heavy chain) combinations determine the unique binding specificity of the antibody produced by each mature B cell. The allelic exclusion principle is reinforced by its outcome: each differentiated B cell and the antibodies it can secrete are confined to a single antigen-binding specificity.