(1) The occupation of receptors by an antagonist so that usual agonists are relatively ineffective. (2) Receptor blockade, blocking the effect of a hormone at the cell surface. (3) Arrest of nerve impulse conduction or transmission at autonomic synaptic junctions, autonomic receptor sites, or neuromuscular junctions by various means, most often pharmacotherapy. (4) Intravenous injection of large amounts of colloidal dyes or other substances to block reticuloendothelial cells (e.g., phagocytosis is temporarily prevented).