Recently, antibody-based fluorescent biosensors are receiving considerable attention as a suitable biomolecule for diagnostics, namely, homogeneous immunoassay and also as an imaging probe. To date, several strategies for "reagentless biosensors" based on antibodies and natural and engineered binding proteins have been described. In this review, several approaches are introduced including a recently described fluorescent antibody-based biosensor Quenchbody, which works on the principle of fluorescence quenching of attached dye and its antigen-dependent release. The merits and possible demerits of each approach are discussed. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody. (C) 2014 Elsevier B.V. All rights reserved.