Which of the following best describes a sensitizer?

A sensitizer is defined as a chemical that can induce an allergic reaction following repeated exposure, even if the initial exposure does not cause any adverse effects. This means that the sensitizer alters the immune system in a way that it reacts against the chemical upon subsequent contacts, leading to potential symptoms such as rashes, breathing difficulties, or other allergic responses.

This characteristic differentiates sensitizers from other types of chemicals, making the ability to produce an allergic response after exposure a critical identifying feature. In the context of hazardous materials, understanding sensitizers is vital, as they can pose significant health risks to workers and responders who may not immediately be aware of their sensitivities to certain substances.

The other descriptions do not accurately capture the essence of sensitizers, focusing instead on aspects like irritation or permanent toxicity, which do not encompass the mechanism of sensitization properly.