Definition
Hypersensitivity reactions are excessive or inappropriate immune responses to antigens.
These antigens can be harmless environmental substances (e.g., pollen, food) or self-antigens (as seen in autoimmune diseases).
There are four types of hypersensitivity reactions, classified based on their immune mechanism and time course:
Type | Immune Mechanism | Mediators | Examples |
Type I (Immediate) | IgE-mediated | Histamine, mast cells, basophils | Allergies, Anaphylaxis, Asthma |
Type II (Cytotoxic) | IgG/IgM-mediated | Complement, ADCC (Antibody-Dependent Cellular Cytotoxicity) | Hemolytic anemia, Myasthenia gravis |
Type III (Immune Complex-Mediated) | Immune complex deposition | Complement, Neutrophils | Lupus, Serum sickness |
Type IV (Delayed-Type) | T-cell mediated | Cytokines, Macrophages | Tuberculosis, Contact dermatitis |
Types of Hypersensitivity Reactions
1) Type I (Immediate or Anaphylactic Hypersensitivity)
Mediated by IgE bound to mast cells and basophils.
Rapid onset after allergen exposure.
Mechanism:
Initial exposure → B cells produce IgE, which binds to mast cells.
Re-exposure → Cross-linking of IgE → Mast cell degranulation → Release of histamine, leukotrienes, prostaglandins → Symptoms.
Examples:
Allergic rhinitis (hay fever)
Asthma
Anaphylaxis
Hives (urticaria)
2) Type II (Cytotoxic Hypersensitivity)
Mediated by IgG or IgM directed against host cell surfaces or extracellular matrix.
Mechanism:
Antibody binding activates complement or triggers phagocytosis, leading to cell destruction.
Examples:
Autoimmune hemolytic anemia (antibodies attack RBCs).
Goodpasture syndrome (antibodies bind kidney/lung basement membrane).
Myasthenia gravis (antibodies block acetylcholine receptors).
3) Type III (Immune Complex Hypersensitivity)
Caused by immune complexes (antigen-antibody complexes, typically IgG) that deposit in tissues, triggering inflammation via complement activation.
Examples:
Serum sickness (reaction to foreign proteins).
Arthus reaction (localized immune complex deposition in the skin).
Vasculitis (e.g., systemic lupus erythematosus, SLE).
4) Type IV (Delayed-Type or Cell-Mediated Hypersensitivity)
Mediated by T cells (CD4⁺ Th1 and/or CD8⁺ cytotoxic T cells).
Onset: 48–72 hours after antigen exposure.
Examples:
Contact dermatitis (e.g., poison ivy, nickel allergy).
Tuberculin skin test (Mantoux test).
Chronic transplant rejection.
