Hypersensitivity, commonly known as allergy could be defined as an exaggerated response of immune system towards certain allergens present in environment. allergen are any substance that cause such vigorous immune response. For example: Pollens, dust mites can be considered as common examples of allergens. Symptoms of allergic reactions include: running nose, watery eyes, sneezing etc. Allergy is caused due to release of certain chemicals like histamine and serotonin from mast cells.