On normally encountering a foreign antigen, an immune system manifests a numbers of responses to combat this challenge. Sometimes, there is an increased or inappropriate immune response to antigenic challenge known as hypersensitivity or allergy.
The agents that induce allergic reaction are called as allergens viz. certain foods like milk, eggs, shell-fish, etc.; antibiotics viz. pencillin, tetracycline, etc., vaccines venoms (honeybee, wasp, snake), lens, dust, iodine-containing dyes, etc.
On re-entry of these allergens into the body to which a person is "sensitilised", one produces IgE antibodies which binds on surface of "mast" cells and basophils.These cells release chemicals like histamine, prostaglandins, etc. which are carried by the blood stream to skin or mucous membrane, and produce symptoms of allergy viz. sneezing, sniffing, gasping, running eyes, inflammation of skin in form of dermatitis, eczema, etc.
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