ABSTRACT
Different microbes have different biological properties and can enter the body through different routes (the skin, the gastrointestinal tract, the respiratory tract, or the genitouri-nary tract). It is likely that the five different antibody classes (IgM, IgD, IgG, IgE, and IgA; Figure 1) and their subclasses have evolved at least partly to facilitate protection against microbes entering at the different sites and with different properties. There is some overlap in their function and in where they are produced, but generally there is a division of labor among the different antibody classes: for example, IgA is the most common antibody in mucosal secretions while IgM is mainly found in the plasma, and both are most effective in those locations.
