Why are there different antibody isotypes in our immune system?

In our immune system, the different antibody isotopes—IgA, IgD, IgE, IgG, and IgM—are classed based on the type of heavy chain they contain, and differ in structure and function. IgA prevents colonization of pathogens in mucosal areas such as the respiratory tract and the gut. IgE binds to allergens and triggers the release of histamine and other factors. IgM eliminates pathogens in the primary immune response, while IgD works with IgM as an antigen receptor on B cells. IgG works to provide the majority of immunity against pathogens in the secondary immune response; it is also the predominant immunoglobin in serum.