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Allergy & Hypersensitivity for USMLE Step 1

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Allergy for the USMLE Step 1 Exam
Allergy Overview
  • Definition:
    • Allergies are hypersensitivity reactions of the immune system to otherwise harmless substances (allergens), leading to symptoms ranging from mild discomfort to severe anaphylaxis.
    • Primarily mediated by IgE antibodies and activation of mast cells and basophils.
  • Common Allergens:
    • Environmental: Pollens, dust mites, animal dander, and molds.
    • Food: Includes peanuts, shellfish, tree nuts, eggs, and milk.
    • Insect Stings: Hymenoptera (e.g., bees, wasps).
    • Medications: Commonly penicillin and NSAIDs.
    • Occupational: Chemicals and latex in certain professions.
Mechanisms of Hypersensitivity
  • Type I (IgE-Mediated) Hypersensitivity:
    • Mechanism: Sensitization on initial exposure leads to IgE antibody production. Upon re-exposure, IgE cross-links on mast cells, causing degranulation and release of histamine and other mediators.
    • Examples: Allergic rhinitis, asthma, food allergies, and anaphylaxis.
  • Type II (Cytotoxic) Hypersensitivity:
    • Mechanism: IgG or IgM antibodies target cell-bound antigens, activating the complement system and causing cell lysis.
    • Examples: Hemolytic reactions, such as autoimmune hemolytic anemia.
  • Type III (Immune Complex) Hypersensitivity:
    • Mechanism: Antigen-antibody complexes form and deposit in tissues, triggering inflammation.
    • Examples: Serum sickness, systemic lupus erythematosus (SLE).
  • Type IV (Delayed-Type) Hypersensitivity:
    • Mechanism: T-cell mediated response, usually occurring 24-72 hours after exposure.
    • Examples: Contact dermatitis and tuberculosis skin test reaction.
Clinical Presentations
  • Allergic Rhinitis:
    • Symptoms: Sneezing, nasal congestion, rhinorrhea, itchy eyes and nose; seasonal or perennial based on allergen.
    • Diagnosis: Clinical, supported by skin or serum IgE testing.
    • Treatment: Intranasal corticosteroids, antihistamines, and allergen avoidance.
  • Asthma:
    • Symptoms: Wheezing, cough, shortness of breath, and chest tightness, often triggered by allergens.
    • Diagnosis: Based on pulmonary function tests (PFTs); IgE testing for specific triggers.
    • Treatment: Inhaled corticosteroids, bronchodilators, and leukotriene receptor antagonists.
  • Atopic Dermatitis:
    • Symptoms: Pruritic, erythematous rash on flexural surfaces.
    • Diagnosis: Clinical with potential allergy testing if food or environmental triggers are suspected.
    • Treatment: Emollients, topical corticosteroids, and avoidance of known allergens.
  • Food Allergy:
    • Symptoms: Urticaria, angioedema, GI symptoms (e.g., vomiting, diarrhea), and potentially anaphylaxis.
    • Diagnosis: Skin prick or serum IgE testing; oral food challenge in uncertain cases.
    • Treatment: Avoidance and epinephrine autoinjectors for severe reactions.
  • Drug Allergy:
    • Symptoms: Ranges from mild rash to severe anaphylaxis.
    • Diagnosis: Based on history, and skin testing if available (e.g., penicillin).
    • Treatment: Avoidance of the drug; desensitization if necessary.
  • Anaphylaxis:
    • Symptoms: Acute onset of systemic symptoms, including urticaria, hypotension, bronchospasm, and shock.
    • Diagnosis: Clinical, based on history and presentation.
    • Treatment: Immediate intramuscular epinephrine, followed by antihistamines and corticosteroids. Patients should carry an epinephrine autoinjector.
Diagnostic Tests
  • Skin Prick Testing:
    • Mechanism: Small amounts of allergens are pricked into the skin; a wheal-and-flare response indicates sensitization.
    • Applications: Diagnosis of allergic rhinitis, asthma triggers, and food allergies.
Allergy Skin Test
  • Serum IgE Testing:
    • Mechanism: Measures allergen-specific IgE antibodies in blood.
    • Applications: Useful when skin testing is not possible, such as in patients with extensive eczema.
  • Oral Food Challenge:
    • Procedure: Gradual exposure to suspected allergen under medical supervision.
    • Applications: Gold standard for confirming food allergies, especially when other tests are inconclusive.
Treatment Overview
  • Avoidance: Primary strategy for managing known allergens (e.g., dietary restrictions, allergen-proofing homes).
  • Pharmacologic:
    • Antihistamines: First-line for mild allergies, especially allergic rhinitis.
    • Corticosteroids: Used for inflammation control in allergic rhinitis, asthma, and dermatitis.
    • Bronchodilators: For acute asthma management.
  • Immunotherapy:
    • Subcutaneous and Sublingual Immunotherapy: Long-term treatment for allergic rhinitis and some cases of asthma.
    • Indications: Patients with severe allergic rhinitis or asthma not controlled by standard treatments.
Key Points
  • Allergies involve hypersensitivity reactions, primarily IgE-mediated, causing symptoms ranging from mild (e.g., rhinitis) to life-threatening (e.g., anaphylaxis).
  • Common allergens include environmental triggers, foods, and certain medications.
  • Clinical presentation varies widely, including respiratory symptoms (rhinitis, asthma), dermatologic manifestations (atopic dermatitis), and systemic reactions (anaphylaxis).
  • Diagnosis relies on history, clinical examination, and confirmatory testing (e.g., skin prick, serum IgE, food challenges).
  • Treatment is allergen-specific and includes avoidance, pharmacologic management (antihistamines, corticosteroids), and immunotherapy in selected cases.
  • Anaphylaxis requires immediate administration of epinephrine, and patients at risk should carry an epinephrine autoinjector.

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