allergic reaction, an unfavorable physiological response to an allergen to which a person has previously been exposed and to which the person has developed antibodies. The response may be characterized by a variety of symptoms, including urticaria, eczema, dyspnea, bronchospasm, diarrhea, rhinitis, sinusitis, laryngospasm, and anaphylaxis. Allergic reactions may be immediate or delayed. Eosinophilia is usually present and is revealed in the differential white blood cell count.
