noun

1. A disorder of the immune system causing adverse reactions to substances (allergens) not harmful to most and marked by the body's production of histamines and associated with atopy, anaphylaxis, and asthma.

2. Any condition of hypersensitivity to a substance.

3. Altered susceptibility to a first treatment as exhibited in reaction to a subsequent one.

4. An antipathy, as toward a person or activity.