See:
Mixtures of similarly acting compounds in Daphnia magna: From gene to metabolite and beyondTine Vandenbrouck, Oliver A.H. Jones, Nathalie Dom, Julian L. Griffin, Wim De CoenEnvironment International 36 (2010) 254-268
The biological role played by a material entity when bound by a receptor of the adaptive immune system. Specific site on an antigen to which an antibody binds.