Recent outbreaks of severe acute respiratory syndrome and Middle East respiratory syndrome， along with the threat of a future coronavirus-mediated pandemic， underscore the importance of finding ways to combat these viruses. The trimeric spike transmembrane glycoprotein S mediates entry into?host cells and is the major target of neutralizing antibodies. To understand the humoral immune response elicited upon natural infections with coronaviruses， we structurally characterized the SARS-CoV and MERS-CoV S glycoproteins in complex with neutralizing antibodies isolated from human survivors. Although the two antibodies studied blocked attachment to the host cell receptor， only the anti-SARS-CoV S antibody triggered fusogenic conformational changes via receptor functional mimicry. These results provide a structural framework for understanding coronavirus neutralization by human antibodies and shed light on activation of coronavirus membrane fusion， which takes place through a receptor-driven ratcheting mechanism.