(a) An allergy is an exaggerated response of the body's immune system to a normally harmless substance, such as pollen, food or house dust, mite or certain antigens present in the environment. The substances which cause such responses are called allergens.
(b) The allergens stimulate the immune system to produce lgE antibodies and release of chemicals like histamine and serotonin from mast cells which produce symptoms of allergy. Allergy can cause :
(i) Runny nose or itchy eyes and mouth.
(ii) Skin rash and gastrointestinal symptoms such as nausea and vomiting.