Model Answer
0 min readIntroduction
Hypersensitivity refers to an exaggerated or inappropriate immune response to a normally harmless environmental antigen. These reactions are a maladaptive immune response that causes tissue damage and disease. They are categorized into four main types based on the immune mechanisms involved, initially described by Gell and Coombs in 1968. Understanding hypersensitivity reactions is crucial in clinical medicine as they underlie a wide range of common diseases, from allergies and asthma to autoimmune disorders and transplant rejection. This answer will detail the different types of hypersensitivity reactions, with a specific focus on Type I, its role in health and disease.
Types of Hypersensitivity Reactions
Hypersensitivity reactions are broadly classified into four types:
- Type I: IgE-mediated – Immediate hypersensitivity. Involves IgE antibodies bound to mast cells and basophils.
- Type II: Antibody-mediated cytotoxic hypersensitivity – IgG or IgM antibodies bind to antigens on cell surfaces, leading to cell destruction.
- Type III: Immune complex-mediated hypersensitivity – Formation of immune complexes that deposit in tissues, causing inflammation and damage.
- Type IV: Cell-mediated hypersensitivity – T cell-mediated immune response, often delayed in onset.
Here's a table summarizing the different types:
| Type | Mechanism | Effector Molecules | Time of Reaction | Examples |
|---|---|---|---|---|
| I | IgE binding to mast cells/basophils | IgE, Histamine, Leukotrienes | Immediate (minutes) | Allergic rhinitis, Asthma, Anaphylaxis |
| II | Antibody binding to cell surface antigens | IgG, IgM, Complement | Hours to days | Hemolytic anemia, Goodpasture's syndrome |
| III | Immune complex deposition | IgG, IgM, Complement | Hours to weeks | Serum sickness, Systemic Lupus Erythematosus |
| IV | T cell-mediated | T cells, Cytokines | Delayed (24-72 hours) | Contact dermatitis, Tuberculosis, Graft rejection |
Type I Hypersensitivity Reaction: A Detailed Look
Type I hypersensitivity, also known as immediate hypersensitivity, is triggered by exposure to an allergen. The process involves sensitization and elicitation phases.
Sensitization Phase
- Initial exposure to an allergen stimulates B cells to produce IgE antibodies.
- These IgE antibodies bind to high-affinity FcεRI receptors on the surface of mast cells and basophils.
- This binding primes the mast cells and basophils for subsequent allergen exposure.
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 prostaglandins.
- It also stimulates the synthesis of newly formed mediators like leukotrienes and cytokines.
- These mediators cause the characteristic symptoms of Type I hypersensitivity.
Role in Health and Disease
Role in Health: While generally considered pathological, some degree of IgE-mediated response can be protective against parasitic infections. IgE can activate eosinophils to kill parasites.
Role in Disease: Type I hypersensitivity underlies a variety of allergic diseases:
- Allergic Rhinitis (Hay Fever): Allergen exposure leads to nasal congestion, sneezing, and itching.
- Asthma: Allergen exposure causes bronchoconstriction, inflammation, and mucus production in the airways.
- Urticaria (Hives): Allergen exposure causes itchy, raised welts on the skin.
- Anaphylaxis: A severe, life-threatening systemic reaction characterized by widespread mast cell degranulation, leading to hypotension, bronchospasm, and shock. Common triggers include insect stings, food allergies (peanuts, shellfish), and medications (penicillin).
- Food Allergies: Reactions to specific food proteins, ranging from mild symptoms like hives to severe anaphylaxis.
Conclusion
Hypersensitivity reactions represent a spectrum of immune responses that, while normally protective, can become detrimental when misdirected or exaggerated. Type I hypersensitivity, mediated by IgE, is a common cause of allergic diseases, ranging from mild inconveniences like hay fever to life-threatening anaphylaxis. Understanding the underlying mechanisms of these reactions is crucial for developing effective diagnostic and therapeutic strategies to manage and prevent these conditions. Further research into immune tolerance and allergen-specific immunotherapy holds promise for long-term solutions.
Answer Length
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