The importance of specific antibodies for the clearance of and long-term resistance to many infectious pathogens has long been appreciated. Moreover, the role in these processes of the different antibody heavy chain isotypes, each tailored to induce diverse effector pathways such as those mediated by complement and Fc receptors and to promote antibody localization in distinct regions of the body, is well established. Insights into the molecular mechanism of isotype class switching showed that during an immune response B cells could change the heavy chain of the antibody they produced, without...