_____ is an allergic reaction created by repeated exposure to a chemical or a substance.

• A. Sensitization
• B. Irritation
• C. Allergy
• D. Desensitization

Answer: (A)

Sensitization is the process by which repeated exposure to a chemical trains the immune system to react. At first exposure, you may have no symptoms, but the body builds a specific immune memory (IgE antibodies) to the allergen. When exposure happens again, these antibodies trigger the release of histamine and other mediators from mast cells, producing the allergic reaction. This distinguishes it from irritation, which is a direct chemical effect on tissues rather than an immune-driven response. Desensitization is a treatment approach to reduce sensitivity, not the reaction itself, and allergy refers to the immune reaction as a whole rather than the process of becoming sensitized.