The present invention relates to a Fab fragment based switchable antibody system for generating site-specific antibody conjugates. Methods are described for the attachment of molecules to specific sites the Fab fragment and for the attachment of the Fab fragment to a target molecule (e.g., an antibody) directed against any desired target antigen (tumor, bacterial, fungal, viral, parasitic etc.) The attachment is via binding of the Fab fragment to an epitope linked to the target molecule.