ABSTRACT
In biological sciences, understanding of mechanisms of complex biological processes has resulted from dissection of the system into its component parts. It is now accepted that most controls of complex processes are mediated by the affinity and/or covalent reaction of specific substances with particular regions in macromolecules. These areas are now called receptors, which may be proteins with or without catalytic functions, lipoproteins, glycoproteins or nucleic acids. When intoxication occurs it is now accepted that it is initiated by interactions among particular macromolecular structures; in some cases these may be pharmacological receptors but not always. Therefore to make a distinction between beneficial (e.g. for hormones and drugs) and undesirable consequences (e.g. for toxic chemicals and side-effects of drugs), what the macromolecules interact with may be called receptors and targets respectively; sometimes reactions with receptors can cause toxicity, i.e. the receptor is the target.
