Final answer:
Isotypic differences in antibodies are defined by their constant regions, which determine their individual classes or isotypes, such as IgM, IgG, IgA, IgD, and IgE.
Step-by-step explanation:
The isotypic differences between antibodies are defined by their constant regions. The five classes of antibodies (IgM, IgG, IgA, IgD, IgE) have unique heavy chains and constant domains that determine their immunological mechanism of action. Within each class, antibodies have the same number of constant domains, which differentiates the classes from one another and affects their functional roles in the immune response. The variable regions of the heavy and light chains are what interact to form the binding site, allowing an antibody to bind to specific epitopes on an antigen. However, it is the constant region that determines the isotype of an antibody.