Allergic reactions are categorized into several types based on their clinical features and mechanisms.
Main types:
Type I Hypersensitivity (Immediate Hypersensitivity): Rapid and immediate allergic reactions occur when an allergen interacts with IgE antibodies on the surface of mast cells and basophils, leading to the release of histamine and other inflammatory mediators. Examples: Allergic rhinitis (hay fever), asthma, anaphylaxis, allergic conjunctivitis, and food allergies.
Type II Hypersensitivity (Cytotoxic Hypersensitivity). Mechanism: Antibodies (IgG or IgM) bind to antigens on the surface of cells, leading to cell destruction by complement activation or phagocytosis. Examples: Hemolytic transfusion reactions, autoimmune hemolytic anemia, and certain drug-induced immune reactions.
Type III Hypersensitivity (Immune Complex-Mediated Hypersensitivity): Immune complexes, formed by the interaction of antigens and antibodies (IgG or IgM), deposit in tissues and cause inflammation by activating complement and attracting immune cells. Examples: Systemic lupus erythematosus, rheumatoid arthritis, and some drug reactions like serum sickness.
Type IV Hypersensitivity (Delayed-Type Hypersensitivity): This reaction is cell-mediated and involves T cells, not antibodies. It typically develops over hours to days after exposure to the allergen. Examples: Contact dermatitis (e.g., poison ivy or poison oak), tuberculosis skin test reactions, and graft rejection.
Type V Hypersensitivity (Stimulatory Hypersensitivity): In this rare type, antibodies stimulate receptor function rather than blocking it. Example: Graves’ disease.