- Hypersensitivity reactions are excessive or inappropriate immune responses that can damage tissues and cause disease. These reactions are classified into four main types based on their immunological mechanisms and timing.
- Type I (Immediate) hypersensitivity involves IgE-mediated reactions, occurring within minutes to hours of exposure. When allergen-specific IgE antibodies bound to mast cells encounter their specific allergen, the cells degranulate, releasing histamine and other inflammatory mediators. This leads to typical allergic symptoms like sneezing, itching, and bronchial constriction. Common examples include allergic rhinitis, asthma, food allergies, and anaphylaxis. The severity can range from mild discomfort to life-threatening reactions.
- Type II (Cytotoxic) hypersensitivity occurs when IgG or IgM antibodies target specific cell surface or matrix antigens. These antibodies can cause cell destruction through complement activation or antibody-dependent cell-mediated cytotoxicity. Examples include autoimmune hemolytic anemia, transfusion reactions, and certain drug reactions. The damage typically develops over hours to days and can affect various tissues and organs.
- Type III (Immune Complex) hypersensitivity results from the formation and deposition of antigen-antibody complexes in tissues. These immune complexes activate complement and recruit inflammatory cells, leading to tissue damage. Systemic lupus erythematosus, post-streptococcal glomerulonephritis, and serum sickness are classic examples. Symptoms typically develop over several days as complexes accumulate in tissues.
- Type IV (Delayed) hypersensitivity is mediated by T cells rather than antibodies and takes 24-72 hours to develop. When sensitized T cells encounter their specific antigen, they release cytokines that recruit and activate other inflammatory cells. This type includes contact dermatitis, tuberculin reactions, and many autoimmune conditions. The resulting inflammation can cause significant tissue damage if not properly controlled.
