Hypersensitivity reactions are classified into four main types, each characterized by different immune mechanisms and types of immune response. Below is a summary of each hypersensitivity type along with associated disorders:
Type I Hypersensitivity (Immediate or Anaphylactic Reaction)
- Mechanism: Involves IgE antibodies reacting with allergens, leading to mast cell degranulation and release of histamines and other mediators.
- Disorders:
- Allergic rhinitis (hay fever)
- Asthma
- Food allergies (e.g., peanut allergy)
- Allergic conjunctivitis
- Anaphylaxis (severe systemic reaction)
Type II Hypersensitivity (Antibody-Mediated Cytotoxic Reaction)
- Mechanism: Involves IgG or IgM antibodies binding to antigens on cell surfaces, leading to complement activation or cytotoxicity.
- Disorders:
- Hemolytic anemia
- Goodpasture syndrome
- Myasthenia gravis
- Graves' disease (hyperthyroidism)
- Rh incompatibility (hemolytic disease of the newborn)
Type III Hypersensitivity (Immune Complex-Mediated Reaction)
- Mechanism: Involves the formation of immune complexes (antigen-antibody complexes) that deposit in tissues, leading to inflammation and tissue damage.
- Disorders:
- Systemic lupus erythematosus (SLE)
- Rheumatoid arthritis
- Serum sickness
- Arthus reaction
- Vasculitis (e.g., polyarteritis nodosa)
Type IV Hypersensitivity (Delayed-Type or Cell-Mediated Reaction)
- Mechanism: Involves T-cell mediated responses rather than antibodies; delayed reaction due to the activation of T cells.
- Disorders:
- Contact dermatitis (e.g., poison ivy)
- Tuberculosis (e.g., tuberculin skin test reaction)
- Graft-versus-host disease
- Type 1 diabetes (autoimmune)
- Multiple sclerosis (autoimmune)
These classifications help in understanding the mechanisms behind various allergic and autoimmune disorders to guide proper diagnosis and treatment.