Which hazard characteristic causes a severe immune reaction after repeated exposure?

• A. Toxic
• B. Sensitizer
• C. Explosive
• D. Oxidizer

Answer: (B)

A sensitizer is a substance that trains the immune system to react against it. On first contact, the immune system may not show a strong reaction, but it becomes sensitized. After that, subsequent exposures trigger a much stronger immune response, which can be severe and manifest as allergic contact dermatitis, asthma, or other hypersensitivity symptoms. This immune-memory effect is what makes repeated exposure dangerous.

This differs from a purely toxic substance, whose harm comes from direct chemical toxicity and dose, not immune sensitization. It also isn’t about substances that cause explosions or that simply support combustion; those hazards relate to physical or energetic properties, not immune reactions.