We know how drugs pass into and out of the body, but the mode of action of many of them is still unclear. It seems that some drugs interact with special cell or tissue elements to produce their characteristic pharmacological effects. The receptors involved appear to be proteins, enzymes, lipoproteins and nucleic acids. The complex reaction between a receptor and drug is thought to trigger events that alter the biological systems of the body. This could include inhibition of an enzyme, release of a neurotransmitter, or interference with absorption of chemicals in the kidneys. When drugs interact with a receptor, which in turn elicits a biological response, they are known as agonists. Sometimes it is necessary to administer a drug to block the attachment of substances to receptors, and such drugs are known as antagonists. They are used to counteract the effects of some drugs. Antagonists do not elicit a biological response by their attachment to receptors.