Model Answer
0 min readIntroduction
Type I hypersensitivity, also known as immediate hypersensitivity or allergy, represents a rapid immunological reaction occurring within minutes of exposure to an allergen. It is a common immune response affecting a significant portion of the population, ranging from mild conditions like allergic rhinitis to life-threatening anaphylaxis. This reaction is mediated by immunoglobulin E (IgE) antibodies and mast cell activation, leading to the release of various inflammatory mediators. Understanding the underlying mechanisms and appropriate management strategies is crucial for effective patient care.
Mechanism of Type I Hypersensitivity Reaction
The Type I hypersensitivity reaction unfolds in two phases: sensitization and elicitation.
Sensitization Phase
- Allergen Exposure: Initial exposure to an allergen (e.g., pollen, dust mites, food proteins) doesn't typically cause a significant reaction.
- IgE Production: Antigen-presenting cells (APCs), like dendritic cells, process the allergen and present it to T helper 2 (Th2) cells. Th2 cells release cytokines like IL-4 and IL-13, which stimulate B cells to differentiate into plasma cells.
- IgE Binding: These plasma cells produce large amounts of allergen-specific IgE antibodies. IgE antibodies then bind to high-affinity FcεRI receptors on the surface of mast cells and basophils. This completes the sensitization phase.
Elicitation Phase
- Re-exposure to Allergen: Upon subsequent exposure to the same allergen, it cross-links the IgE antibodies bound to FcεRI receptors on mast cells and basophils.
- Mast Cell/Basophil Activation: This cross-linking triggers the degranulation of mast cells and basophils.
- Mediator Release: Degranulation releases a variety of preformed and newly synthesized mediators, including:
- Histamine: Causes vasodilation, increased vascular permeability, and bronchoconstriction.
- Heparin: Anticoagulant.
- Proteases: Contribute to tissue damage.
- Leukotrienes & Prostaglandins: Sustained inflammatory response, bronchoconstriction, and mucus production.
- Cytokines: Recruit other inflammatory cells.
- Clinical Effects: These mediators cause the characteristic symptoms of Type I hypersensitivity, ranging from localized reactions (e.g., hives, itching) to systemic reactions (e.g., anaphylaxis).
Agents Inducing Type I Hypersensitivity
Numerous agents can induce Type I hypersensitivity reactions. These are broadly categorized as allergens.
- Inhalants: Pollen (grass, ragweed, trees), dust mites, mold spores, animal dander.
- Ingestants: Foods (peanuts, tree nuts, milk, eggs, shellfish, soy, wheat), food additives, medications.
- Injectables: Insect venoms (bees, wasps, ants), drugs (penicillin, antibiotics), vaccines (rarely).
- Contactants: Latex, certain chemicals.
The potency of an allergen depends on factors like its ability to bind to IgE, its persistence in the environment, and the individual's genetic predisposition.
Laboratory Evaluation and Management of Allergic Reactions
Laboratory Evaluation
- Skin Prick Tests (SPT): Small amounts of suspected allergens are pricked into the skin. A wheal and flare reaction indicates sensitivity.
- Intradermal Tests: Allergen is injected intradermally. More sensitive than SPT but carries a higher risk of systemic reaction.
- Specific IgE Antibody Tests (RAST/ImmunoCAP): Measures the amount of IgE antibody specific to a particular allergen in the serum.
- Complete Blood Count (CBC): May show eosinophilia (increased eosinophils) in allergic conditions.
- Total Serum IgE: Elevated levels can suggest an allergic predisposition, but is not specific.
Management of Allergic Reactions
- Avoidance: The most effective strategy is to avoid the allergen.
- Pharmacological Treatment:
- Antihistamines: Block histamine receptors, relieving symptoms like itching, sneezing, and runny nose.
- Decongestants: Reduce nasal congestion.
- Corticosteroids: Reduce inflammation (oral, inhaled, topical).
- Epinephrine: Used for anaphylaxis; reverses airway obstruction, hypotension, and angioedema.
- Leukotriene Receptor Antagonists: Block the effects of leukotrienes.
- Immunotherapy (Allergen Immunotherapy): Involves repeated injections of increasing doses of the allergen to desensitize the individual. Effective for allergies like pollen, dust mites, and insect venom.
- Emergency Management of Anaphylaxis: Immediate epinephrine administration, followed by airway management, oxygen therapy, and intravenous fluids.
Conclusion
Type I hypersensitivity reactions are complex immunological events driven by IgE and mast cell activation. Accurate diagnosis through skin tests and specific IgE assays, coupled with allergen avoidance and appropriate pharmacological or immunotherapeutic interventions, are crucial for managing allergic conditions. The increasing prevalence of allergies globally necessitates continued research into preventative strategies and improved treatment modalities.
Answer Length
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