Abstract: The conserved N-linked glycosylation site on the Fc domain of IgG1 antibodies is essential for maintaining a functionally active conformation of the antibody. Different glycoforms of the Fc exhibit widely different effector functions. Similarly, therapeutic antibodies, with engineered glycosylation, exhibit altered binding to cellular Fc receptors (FcRs). Here, X-ray crystallographic structures were obtained for biosynthetic intermediate glycoforms of human IgG1 Fc bearing: unprocessed oligomannose-type, intermediate hybrid-type, and mature complex-type glycans. The fully processed Fc protein crystallised in an “open” conformation with glycans forming canonical stabilising interactions on the protein surface. Analysis of the biosynthetic intermediates revealed that these stabilising hydrophobic protein-glycan interactions are formed only after processing by Golgi -mannosidase II. Mutagenesis of hydrophobic residues on Fc disrupted crucial protein-glycan interactions resulting in the selective destabilization of the 3-arm of the glycan chain with the 6-arm closely matching that seen for the native structure. However, carbohydrate analysis of released glycans shows increased processing on both arms indicating a more accessible and flexible glycan in the mutant structure suggesting that the crystallographic structure of these antibody glycans represents a minor low-energy conformation. The importance of Fc glycosylation is highlighted by endoglycosidases which eliminate Fc effector function. The crystallographic structure of enzymatically deglycosylated IgG Fc revealed a significant collapse of the of Cγ2 domains resulting in a ‘closed’ quaternary conformation, incompatible with Fc receptor binding. This provides a structural explanation for immune deactivating properties of endoglycosidases including those under preclinical development for the treatment of antibody-mediated immune pathology. One such bacterial endoglycosidase, Endo S, was studied further and revealed a specificity for complex-type glycans of the type found on IgG but no hydrolytic activity towards an engineered IgG Fc with oligomannose-type glycans. Introduction of both the engineered monoclonal IgG and endoglycosidase in serum led to a dramatic increase in FcR binding as the competitive binding of serum IgG for FcRs was selectively eliminated. This approach is a general technique for boosting the effector signal of therapeutic antibodies.