More than twenty recombinant antibody molecules are now licensed for the treatment of a variety of cancers and chronic diseases. Initially, the attraction of antibodies was their specificity for target antigens; however, it is now appreciated that the downstream consequences of engaging antigen, after the formation of immune complexes, is crucial to clinical outcomes in vivo. This review introduces the structural and functional activities of the IgG class of recombinant antibodies, in vitro, and criteria that determine choice between the four subclasses. Importantly, we demonstrate that, although accounting for only 2-3% of antibody mass, glycosylation of the IgG-Fc is essential to the activation of downstream biologic mechanisms (effector functions). Additionally the precise structure of the attached oligosaccharide can influence biologic efficacy. These findings have led to cellular engineering to enable the production of selected glycoforms of antibody that are considered to be optimal for the disease indication to be treated.