In research, an antagonist is a molecule that binds to a receptor and inhibits or blocks its biological activity. Antagonists are often used in pharmacology to study the function of receptors or as therapeutic agents to block the activity of certain receptors in the treatment of diseases. Antagonists can be classified as competitive or non-competitive, depending on how they interact with the receptor. They are important tools in drug development and research to understand the mechanisms of biological pathways and signaling.