Model Answer
0 min readIntroduction
Hypersensitivity refers to an exaggerated immune response to a normally harmless environmental antigen, leading to tissue damage and disease. These reactions are categorized based on the immune mechanisms involved, providing a framework for understanding their pathogenesis and clinical presentation. The widely accepted classification system is the Gell and Coombs classification, which divides hypersensitivity reactions into four types. Understanding these types is fundamental to diagnosing and managing allergic and autoimmune conditions. This answer will define hypersensitivity, classify it according to Gell and Coombs, and then provide a detailed description of Type I hypersensitivity.
Defining and Classifying Hypersensitivity
Hypersensitivity reactions are inappropriate and excessive immune responses. They are not the same as autoimmunity, which involves a response against self-antigens. The Gell and Coombs classification categorizes hypersensitivity into four types based on the immune mechanism driving the reaction:
| Type | Mechanism | Mediator | Examples |
|---|---|---|---|
| Type I (Immediate) | IgE-mediated | Mast cell & Basophil degranulation | Allergic rhinitis, Asthma, Anaphylaxis |
| Type II (Cytotoxic) | IgG or IgM-mediated | Complement activation, Antibody-dependent cell-mediated cytotoxicity (ADCC) | Transfusion reactions, Hemolytic disease of the newborn |
| Type III (Immune Complex) | Immune complex deposition | Complement activation, Neutrophil recruitment | Serum sickness, Systemic Lupus Erythematosus |
| Type IV (Delayed-Type) | T cell-mediated | Cytokines, Lymphokines | Contact dermatitis, Tuberculin skin test |
Type I Hypersensitivity: A Detailed Description
Type I hypersensitivity, also known as immediate hypersensitivity, is mediated by IgE antibodies. It occurs in two phases: sensitization and elicitation.
Sensitization Phase
- Initial exposure to an allergen (antigen) triggers a response by Th2 helper cells.
- Th2 cells release cytokines like IL-4 and IL-13, which stimulate B cells to differentiate into plasma cells.
- These plasma cells produce IgE antibodies specific to the allergen.
- IgE antibodies bind to high-affinity FcεRI receptors on the surface of mast cells and basophils.
Elicitation Phase
- Subsequent exposure to the same allergen cross-links the IgE antibodies bound to mast cells and basophils.
- This cross-linking triggers degranulation, releasing preformed mediators like histamine, tryptase, and heparin.
- It also stimulates the synthesis and release of newly formed mediators like leukotrienes and prostaglandins.
Mediators and their Effects
- Histamine: Vasodilation, increased vascular permeability, bronchoconstriction, itching.
- Leukotrienes: Prolonged bronchoconstriction, increased mucus secretion.
- Prostaglandins: Vasodilation, pain, fever.
Clinical Manifestations
Type I hypersensitivity manifests in various clinical conditions, ranging from mild localized reactions to life-threatening systemic responses:
- Allergic Rhinitis (Hay Fever): Sneezing, runny nose, itchy eyes.
- Asthma: Bronchoconstriction, wheezing, shortness of breath.
- Urticaria (Hives): Itchy, raised welts on the skin.
- Anaphylaxis: A severe, life-threatening systemic reaction characterized by widespread vasodilation, bronchoconstriction, and shock. Common triggers include insect stings, food allergies (peanuts, shellfish), and medications (penicillin).
Diagnosis often involves skin prick tests or specific IgE antibody assays (RAST tests).
Conclusion
Hypersensitivity reactions represent a spectrum of immune responses that, while protective in many contexts, can become detrimental when directed against harmless substances. Type I hypersensitivity, mediated by IgE and mast cell degranulation, is a common and potentially dangerous form of allergic reaction. Understanding the underlying mechanisms is crucial for effective diagnosis, management, and prevention, particularly in the context of rising allergy prevalence globally. Further research into immunomodulatory therapies holds promise for mitigating the impact of these reactions.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.