BP104T • Unit 4Topic 4 of 4⏱ 5 Min Read
4. Gell and Coombs Classification of Hypersensitivity Reactions (Types I - IV)
💡 Core Concept & Clinical Context:
Hypersensitivity refers to exaggerated, inappropriate, or damaging immune responses against harmless environmental antigens (allergens) or self-antigens, causing tissue damage, inflammation, or death. Gell and Coombs (1963) established the definitive four-tier classification system based on underlying immunological mechanisms.
Clinical Scenarios & Pharmacopoeial Examples:
1Detailed Gell and Coombs Hypersensitivity Mechanisms
Etiology, mediators, and clinical examples.
Step-by-Step Breakdown:
•1. Type I: Immediate / Anaphylactic Hypersensitivity:
•• Mechanism: Allergen cross-links IgE bound to Fc-epsilon-RI receptors on sensitized Mast Cells/Basophils -> Instant degranulation releasing Histamine, Leukotrienes (LTC4, LTD4), and Prostaglandin D2.
•• Onset: Seconds to minutes.
•• Clinical Examples: Systemic Anaphylaxis (Penicillin allergy, bee venom), Allergic Asthma, Hay fever, Urticaria.
•2. Type II: Cytotoxic / Antibody-Mediated Hypersensitivity:
•• Mechanism: IgG or IgM antibodies bind directly to cell-surface antigens -> Activates Complement-mediated lysis (MAC) or Antibody-Dependent Cellular Cytotoxicity (ADCC).
•• Clinical Examples: ABO Incompatible Blood Transfusion Hemolysis, Hemolytic Disease of the Newborn (Rh Incompatibility / Erythroblastosis Fetalis), Autoimmune Hemolytic Anemia, Myasthenia Gravis.
•3. Type III: Immune Complex-Mediated Hypersensitivity:
•• Mechanism: Soluble antigen-antibody (IgG/IgM) immune complexes deposit in vascular basement membranes -> Activates complement -> Attracts Neutrophils that release lysosomal enzymes causing vasculitis and tissue necrosis.
•• Clinical Examples: Systemic Lupus Erythematosus (SLE), Post-Streptococcal Glomerulonephritis, Serum Sickness, Arthus Reaction.
•4. Type IV: Delayed-Type / Cell-Mediated Hypersensitivity (DTH):
•• Mechanism: T-cell mediated (Antibody-independent). Sensitized CD4+ Th1 cells release IFN-gamma and cytokines, activating Macrophages; or CD8+ CTLs directly destroy target cells.
•• Onset: 24 to 72 hours.
•• Clinical Examples: Tuberculin Mantoux Skin Test, Contact Dermatitis (Poison ivy, Nickel allergy), Graft vs Host Disease.
Comparison & Reference Table:
| Type | Descriptive Name | Key Immune Mediators | Onset Speed | Prototypic Clinical Examples |
|---|---|---|---|---|
| Type I | Immediate / Anaphylactic | IgE, Mast cells, Histamine, Leukotrienes | 2 - 30 minutes | Penicillin anaphylaxis, Asthma, Urticaria |
| Type II | Cytotoxic / Tissue-specific | IgG, IgM, Complement, Macrophages | Hours to days | Rh incompatibility, Transfusion reaction, Myasthenia |
| Type III | Immune Complex | Antigen-Antibody complexes, Neutrophils | 3 - 8 hours | SLE, Serum sickness, Rheumatoid arthritis |
| Type IV | Delayed / Cell-Mediated | Sensitized T-cells (Th1, CD8+), Macrophages | 24 - 72 hours | Tuberculin skin test, Contact dermatitis (Nickel) |
📋Key Takeaways & Exam Highlights
- Types I, II, and III are Immediate and strictly Antibody-mediated.
- Type IV is Delayed (24-72 hours) and strictly T-Cell-mediated.
- Mnemonic (ACID): Type I = Anaphylactic / Allergic, Type II = Cytotoxic, Type III = Immune Complex, Type IV = Delayed.
🇮🇳 हिन्दी व्याख्या (4. Gell and Coombs Classification of Hypersensitivity Reactions (Types I - IV))
हाइपरसेंसिटिविटी 4 प्रकार की होती है (ACID): Type 1 = एनाफिलेक्सिस/एलर्जी (IgE और हिस्टामाइन), Type 2 = साइटोटॉक्सिक (खून चढ़ाने में गड़बड़ी), Type 3 = इम्यून कॉम्प्लेक्स (SLE, गठिया), Type 4 = डिलेड/देरी से होने वाली (24-72 घंटे बाद जैसे ट्यूबरकुलिन टेस्ट)।