Potent human-neutralizing Abs from rare memory B cells of individuals infected with SARS-CoV or MERS-CoV in complex with SARS-CoV S or MERS-CoV S to provide molecular-level information of the mechanism of competitive inhibition of SB attachment to the host receptor.The S230 anti-SARS-CoV Ab also acted by functionally mimicking receptor attachment and promoting  S fusogenic  conformational rearrangements through a racheting mechanism that elucidated the unique nature of the coronavirus membrane fusion activation.