Allergy is defined as a "harmful, increased susceptibility to a specific substance," also known as hypersensitivity.

Types of Allergic Disease

Allergic reactions can result in allergic rhinitis, one of the common manifestations, with usually accompanying respiratory discomfort. Usual cause is inhaling of the allergens in the form of weed, tree and grass pollen, mold spores, house dust, and animal dander.

Asthma, is a more serious form of the disease, results in wheezing and shortness of breath due to a narrowing of the bronchial tubes. Asthma is caused by the sensitivity to certain the allergens.

Skin allergies include eczema, or atopic dermatitis, which is characterised by itchy rashes and oozing blisters; hives or urticaria involving swelling, redness and itching; and contact dermatitis. When hives are accompanied by swelling (angioedema) of the eyelids, tongue, mouth, hands or feet, difficulty in breathing or swallowing can occur.

Allergies to certain foods, most notably, fish, eggs, milk, nuts, and wheat, induce intestinal disturbances and occur frequently in children who often outgrow it after age three.

Anaphylactic shock is the most severe of form allergic reactions, causing swelling of body tissues, vomiting, cramps, and a sudden drop in blood pressure. Anaphylaxis often occurs in persons particularly sensitive to penicillin, stinging insects and shellfish or nuts.