Which is a sensitizer?

• A. Formaldehyde
• B. Acetaminophen (Tylenol)
• C. Carbon monoxide
• D. Acetone

Answer: (A)

Sensitizers are chemicals that can provoke an immune system–mediated allergic reaction after an initial exposure, leading to a reaction upon subsequent exposures. Formaldehyde fits this pattern because it’s reactive enough to bind to skin and other proteins, forming hapten–protein complexes. These complexes are recognized by immune cells, which can trigger an allergic response (such as dermatitis or asthma) when the chemical is encountered again. The other substances don’t typically produce this kind of immune sensitization: acetaminophen can cause hypersensitivity but is not classically categorized as a sensitizer; carbon monoxide affects oxygen transport rather than the immune system; and acetone is a simple solvent with no known sensitizing effect.