Types of Hypersensitivity Reactions

Hypersensitivity reactions are exaggerated immune responses that can lead to tissue damage and various health issues. Understanding the four types—immediate, antibody-dependent, immune complex, and delayed-type—helps us grasp how the immune system can sometimes go awry.

1. Type I (Immediate) Hypersensitivity

   • Mediated by IgE antibodies that bind to allergens, leading to mast cell degranulation.
   • Commonly associated with allergic reactions such as hay fever, asthma, and anaphylaxis.
   • Symptoms occur within minutes of exposure to the allergen due to the release of histamines and other inflammatory mediators.
   • Can be diagnosed through skin prick tests or serum IgE levels.
   • Treatment often involves antihistamines, corticosteroids, or allergen immunotherapy.

2. Type II (Antibody-Dependent) Hypersensitivity

   • Involves IgG or IgM antibodies that target antigens on the surface of cells, leading to cell destruction.
   • Examples include autoimmune hemolytic anemia and transfusion reactions.
   • Mechanisms include complement activation, opsonization, and antibody-dependent cellular cytotoxicity (ADCC).
   • Diagnosis may involve direct Coombs test or indirect Coombs test.
   • Treatment focuses on immunosuppressive therapies or removal of the offending agent.

3. Type III (Immune Complex) Hypersensitivity

   • Characterized by the formation of immune complexes that deposit in tissues, triggering inflammation.
   • Associated with conditions like systemic lupus erythematosus (SLE) and rheumatoid arthritis.
   • Symptoms can include fever, rash, and joint pain, often occurring several hours to days after exposure.
   • Diagnosis may involve serological tests for specific autoantibodies or complement levels.
   • Treatment typically includes corticosteroids and other immunosuppressive agents to reduce inflammation.

4. Type IV (Delayed-Type) Hypersensitivity

   • Mediated by T cells rather than antibodies, leading to a delayed immune response.
   • Commonly seen in contact dermatitis (e.g., poison ivy) and tuberculin reactions (e.g., TB skin test).
   • Symptoms usually appear 24-48 hours after exposure to the antigen.
   • Diagnosis can be made through patch testing or intradermal skin tests.
   • Treatment often involves topical or systemic corticosteroids to manage inflammation.