Hypersensitivity is an abnormal physiological condition in which the immune system overreacts to an antigen or allergen, causing immune diseases including allergies and autoimmunity. There are four types of hypersensitivity reactions, depending on the class of substance that triggers it.

• Type I Hypersensitivity: This is an immediate hypersensitivity reaction that occurs within 24 hours of exposure to the antigen or allergen. It is predominantly mediated by IgE, IgM, and IgG antibodies. Type I reactions usually develop within a few minutes of exposure to an allergen, but can occasionally develop over a few hours. They are mostly mild but can sometimes develop into a severe reaction called anaphylaxis.

• Type II Hypersensitivity: This reaction refers to an antibody-mediated immune reaction in which antibodies (IgG or IgM) are directed against cellular or extracellular matrix antigens with the resultant cellular destruction, functional loss, or damage to tissues.

• Type III Hypersensitivity: This reaction occurs when immune complexes are formed between antigens and antibodies, which then deposit in tissues and cause inflammation and tissue damage.

• Type IV Hypersensitivity: This reaction is also called ‘delayed hypersensitivity reaction’. These reactions can happen from 12 to 72 hours after exposure to the allergen. For example, contact dermatitis, some lung conditions, type I diabetes, and organ transplant rejection.

Symptoms of hypersensitivity depend on the type of reaction and can range from mild to life-threatening. Some of the symptoms or conditions caused by hypersensitivity include allergies (including fish and shellfish, nuts, eggs, wheat, dairy, soy), cat and dog allergies, grass allergy, pollen allergy, dust mite allergy, mold allergy, allergy to some medicines, insect and mite bite allergies, and latex (rubber) allergy.

The evaluation of hypersensitivity includes complete blood cell count, assessment of immunoglobulins, skin prick test, and detection of autoantibodies.