What is a "sensitizer" in the context of toxicology?

A "sensitizer" in toxicology refers specifically to a chemical that has the ability to cause allergic reactions in an individual after repeated exposure. This means that upon initial exposure, an individual may not show any adverse reactions, but subsequent exposures can trigger an immune response due to the body's sensitization to the substance. This immune response can manifest as symptoms such as rashes, itching, or more severe allergic reactions.

The other options describe different concepts: immediate skin irritation is an effect of irritants, enhancing nervous system function is related to neuroactive substances, and hazardous waste with high toxicity pertains to environmental safety and regulations. None of these relate to the specific mechanism of sensitization that is key to defining a sensitizer in toxicological terms.