The pathogenic bacteria Staph. aureus and Strep. pyogenes each express proteolytic enzymes with actions against IgGs of the host. The proteases, gluV8 and IdeS, respectively, inactivate host antibodies by just a single hinge cleavage. Although ADCC and complement functions of IgGs are greatly reduced, this potent bacterial evasion tactic is successfully reversed by a host immune mechanism. The key connection is human anti-hinge (“damage site”) antibodies (HAH).   As background, in a model of S. aureus infection, we found that rabbits readily mount immune responses against the cleavage site induced by gluV8. Actual suppression of in vivo colony growth was afforded by such anti-hinge antibodies. The explanation was that anti-hinge antibodies form a complex with cleaved, targeting Abs on the S. aureus surface to restore effector killing functions aided by a highly efficacious IgG3 component. A parallel strategy for human tumor therapy is envisioned.   HAH Abs are readily demonstrated in individual and pooled IgGs (IVIg). HAH binds to the IdeS-cleaved hinge to restore and amplify functions. Thus, endogenous immunity is poised to act when it encounters IdeS-mediated damage to host IgGs on cell surfaces, particularly cancer cells. Evidence includes 1) restored B-cell clearance by IdeS-cleaved rituximab in non-human primates, 2) enhanced killing when IdeS-pretreated mAbs such as trastuzumab were exposed to cancer cell lines in the presence of IVIg plus human NK cells; 3) restored function to anti-cancer mAbs already bound to target cells and then treated with IdeS in situ before exposure to IVIg and NKs, and 4) recruitment of anti-tumor Abs from IVIg to the cancer cell surface followed by subsequent exposure to IdeS, IVIg and NKs. In the latter case, the identities and specificities of the anti-tumor Abs in IVIg were undetermined but were nonetheless capable of inducing cell death via the IdeS/HAH assemblage.