Pharmacodynamics is the study of how drugs have effects on the body. The most common mechanism is by the interaction of the drug with tissue receptors located either in cell membranes or in the intracellular fluid. The extent of receptor activation, and the subsequent biological response, is related to the concentration of the activating drug (the 'agonist'). This relationship is described by the dose–response curve, which plots the drug dose (or concentration) against its effect. This important pharmacodynamic relationship can be influenced by patient factors (e.g. age, disease) and by the presence of other drugs that compete for binding at the same receptor (e.g. receptor 'antagonists'). Some drugs acting at the same receptor (or tissue) differ in the magnitude of the biological responses that they can achieve (i.e. their 'efficacy') and the amount of the drug required to achieve a response (i.e. their 'potency'). Drug receptors can be classified on the basis of their selective response to different drugs. Constant exposure of receptors or body systems to drugs sometimes leads to a reduced response (i.e. 'desensitization').