Final answer:
The two rules pertaining to constant region genes are allelic exclusion and class switching. Allelic exclusion prevents the expression of multiple antibody specificities from a single B cell, while class switching allows an antibody to change its isotype without altering antigen specificity.
Step-by-step explanation:
In terms of the constant region genes, two important concepts are allelic exclusion and class switching. Allelic exclusion ensures that each B cell expresses antibody molecules of a single antigen specificity by only allowing one allele of the immunoglobulin gene to be active. Class switching, on the other hand, is a process that changes the constant region of the antibody, allowing it to change its function (for example, from IgM to IgG) while keeping the specificity for the antigen unchanged.
The genes involved in coding for the variable regions of antibodies and T cell receptors undergo genetic recombination, such as V(D)J recombination, to create a large variety of antigen-binding sites. The constant domain, however, remains unchanged in all antibodies, thus it does not bind antigen. This antigen-binding diversity is crucial for the immune system to be prepared to react to a wide range of microbial epitopes.1