WHAT IS AFFINITY

The rate of evaporation is given by the intrinsic rate of dissociation of bound molecules from the surface 

FIGURE 1.7 Key compounds synthesized to eliminate the efficacy (burgundy red) and enhance the affinity (green) of histamine for histamine H2 receptors to make cimetidine, one of the first histamine H2 antagonists of use in the treatment of peptic ulcers. Quotation from James Black [10]. 

Once equilibrium has been reached, the rate of adsorption equals the rate of evaporation. Equating [1.1] and [1.2] and rearranging yields  

This is the Langmuir adsorption isotherm in its original form. In pharmacological nomenclature, it is rewritten in the convention 

It is amazing to note that complex processes such as drug binding to protein, activation of cells, and observation of syncytial cellular response should apparently so closely follow a model based on these simple concepts. This was not lost on A. J. Clark in his treatise on drug receptor theory The Mode of Action of Drugs on Cells [4]  

---