Give an account on hypersensitivity or allergic reaction.

Explanation

Certain antigens present in the environment causintensifying responses of the immune system. These responses are called allergies.
The substance that produce immune responses are called aller gens. Antibodies are produced by these allergen Common allergens in the environment are dust, pollen grains, drugs etc. Antihistamines reduce the symptoms of allergy.
Allergy is due to release of chemicals like histamines and serotonins.
Symptoms of allergitic reactions are sneezing, coughing, watering eye, running rose, itching, etc.