Eculizumab-C5 complexes express a C5a neoepitope in vivo: Consequences for interpretation of patient complement analyses

•Antibodies to neoepitopes are crucial tools for detection of complement activation.•Changes of complement proteins without activation can expose neoepitopes.•Conformation change of C5 exposes a C5a neoepitope without C5a fragment formation.•Eculizumab binding to C5 in vivo induces a C5 neoepitope falsely detected as C5a.•Assays claimed to be activation product specific should be used with great caution.