Explain what an agonist is in pharmacology.

An agonist is defined as a substance that binds to a specific receptor and activates it, leading to a biological response. This means that when an agonist interacts with its corresponding receptor, it can mimic the action of a natural signaling molecule (such as a hormone or neurotransmitter) and elicit a physiological effect in the body.

Agonists are crucial in pharmacology as they are often used in therapeutic contexts to stimulate a desired response. For example, medications that act as agonists for certain receptors can be used to alleviate conditions such as asthma by relaxing the bronchial muscles or managing pain by binding to opioid receptors.

Considering the other options, a substance that neutralizes the effects of another refers to an antagonist or a neutralizing agent rather than an agonist. An antagonist specifically inhibits receptor function, counteracting the actions of agonists. Lastly, a compound that prevents any cellular response would not qualify as an agonist, as it does not promote activation or any biological response at the receptor level. Thus, the definition aligns precisely with the role of an agonist as an activator of receptor-mediated responses.