noun

• (pathology, immunology) 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.
• (pathology) Any condition of hypersensitivity to a substance.
• Altered susceptibility to a first treatment as exhibited in reaction to a subsequent one.
• (informal) An antipathy, as toward a person or activity.