The range of allergies is very board and people may react to various foods, drugs, dust, pollens, fabrics, plants, bacteria, animals, heat, sunlight and many other things.
Whenever a foreign material invades the tissues, the body reacts to fight against it. The body reacts to fight against it.
The body produces certain materials called antibodies which combine with the foreign material and render it harmless. But should it enter the body a second time, the antibodies are torn away from the body tissues to attack the substance.
This causes to a chemical substance called histamine to be the released which in turn produce the disorders which are symptoms of an allergy.