The antibody-antigen interaction is characterised by high affinity and high specificity, which makes it an excellent candidate as an analytical tool for selective and sensitive determinations. This biochemical interaction is being increasingly detected by solid-state physical transducers, in devices which are termed ‘biosensors’. The structure of the antibody molecule is central to its binding function. In addition, several techniques are available by which antibodies may be produced. An appreciation of both of these areas is essential in developing an immunosensor. The choice of the appropriate transducer is also important, and this short article also highlights some recent applications involving the use of electrochemical, optical, surface acoustic wave and semiconductor transducers.