Hypersensitivity

Hypersensitivity refers to immune responses that cause tissue injury or disease in response to normally harmless environmental or self-antigens. The concept encompasses four major categories, known as Type I–IV hypersensitivity, defined by Gell and Coombs, though overlap and variants exist.

Type I hypersensitivity is IgE-mediated and involves rapid release of mediators from mast cells and basophils upon allergen exposure. Sensitization occurs on first contact; subsequent encounters trigger symptoms within minutes, ranging from allergic rhinitis and conjunctivitis to bronchial asthma and anaphylaxis. Diagnosis relies on clinical history, skin prick or specific IgE testing, and management emphasizes avoidance, antihistamines, intranasal steroids, and, for anaphylaxis, epinephrine, with consideration of immunotherapy for certain allergens.

Type II hypersensitivity is antibody-mediated cytotoxicity. IgG or IgM binds to antigens on cell surfaces or extracellular matrix, activating complement or engaging effector cells and causing cell injury or death. Examples include autoimmune hemolytic anemia, Goodpasture syndrome, transfusion reactions, and certain drug-induced cytotoxicities. Treatment targets the underlying cause and may include immune modulation or supportive care.

Type III hypersensitivity involves immune complex deposition and complement activation, leading to inflammation at sites such as blood vessels and organs. It can cause serum sickness, Arthus reactions, and various glomerulonephritides or vasculitides associated with immune complex burden.

Type IV hypersensitivity is T cell–mediated and typically delayed, presenting after 24–72 hours. It includes contact dermatitis from poison ivy or metals, tuberculin skin test reactions, and some forms of transplant rejection and autoimmune pathology.

Diagnosis relies on clinical patterns, specific testing (skin tests, serology, biopsy), and management centers on trigger avoidance, pharmacotherapy, and immunomodulation, with emergency treatment for anaphylaxis.