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Allergic Reactions And Hypersensitivity

Discover the hidden triggers and the science behind allergic reactions and hypersensitivity, unraveling the mysteries that could help you better understand your own body's unique responses.

USMLE Guide: Allergic Reactions and Hypersensitivity


Allergic reactions and hypersensitivity are important topics to understand for the United States Medical Licensing Examination (USMLE). This guide aims to provide an overview of allergic reactions, hypersensitivity reactions, and their classification. It will also discuss common allergens, clinical manifestations, diagnostic approaches, and management strategies.

Table of Contents

  1. Introduction
  2. classification of hypersensitivity Reactions
  3. Allergic Reactions
    • Types of Allergens
    • Clinical Manifestations
    • Diagnostic Approaches
    • Management
  4. Hypersensitivity Reactions
  5. Conclusion

Classification of Hypersensitivity Reactions

Hypersensitivity reactions are classified into four types based on the Gell and Coombs classification system. These types help categorize the immune response involved in allergic reactions.

  1. Type I (Immediate Hypersensitivity): IgE-mediated reactions causing immediate allergic responses.
  2. Type II (Cytotoxic Hypersensitivity): Antibody-mediated reactions leading to destruction of target cells or tissues.
  3. Type III (Immune Complex-Mediated Hypersensitivity): Immune complex deposition triggering inflammation and tissue damage.
  4. Type IV (Cell-Mediated Hypersensitivity): T-cell-mediated reactions causing delayed hypersensitivity responses.

Allergic Reactions

Types of Allergens

Allergens are substances that can induce an allergic reaction. Common allergens include:

  • Pollens
  • Dust mites
  • Animal dander
  • Insect venom
  • Certain foods (e.g., peanuts, shellfish, eggs)
  • Medications (e.g., penicillin)

Clinical Manifestations

Allergic reactions can manifest in various ways, including:

  • Allergic rhinitis (hay fever): Sneezing, itching, rhinorrhea, nasal congestion.
  • Allergic conjunctivitis: Itchy, red, swollen eyes.
  • Asthma: Wheezing, shortness of breath, coughing.
  • Urticaria (hives): Pruritic, raised, erythematous wheals.
  • Anaphylaxis: Severe, life-threatening reaction characterized by bronchospasm, hypotension, angioedema, and/or urticaria.

Diagnostic Approaches

To diagnose allergic reactions, the following tests can be utilized:

  • Skin prick test: Measures immediate-type hypersensitivity reactions by introducing allergens to the skin.
  • Serum-specific IgE test: Measures allergen-specific IgE antibodies in the blood.
  • Challenge tests: Used when other tests are inconclusive, involves exposing the patient to a suspected allergen in a controlled environment.


Management of allergic reactions involves:

  • Avoidance of allergens: Educating patients about their specific triggers and how to minimize exposure.
  • Pharmacotherapy: Antihistamines, intranasal corticosteroids, leukotriene receptor antagonists, and bronchodilators may be prescribed based on the clinical presentation.
  • Immunotherapy: Allergen-specific immunotherapy (e.g., subcutaneous or sublingual) can be considered for patients with identified allergens not easily avoidable.

Hypersensitivity Reactions

Types of Hypersensitivity Reactions

Hypersensitivity reactions can be classified as follows:

  • Type I: Immediate hypersensitivity reactions caused by IgE-mediated mechanisms.
  • Type II: Cytotoxic hypersensitivity reactions mediated by antibodies (IgG, IgM) targeting cells or tissues.
  • Type III: Immune complex-mediated hypersensitivity reactions involving immune complex deposition and subsequent inflammation.
  • Type IV: Cell-mediated hypersensitivity reactions primarily involving T-cell responses.

Clinical Manifestations

Hypersensitivity reactions can present with various clinical manifestations, including:

  • Hemolytic anemia (Type II): Destruction of red blood cells due to antibody recognition.
  • serum sickness (Type III): Fever, rash, arthralgia, and lymphadenopathy due to immune complex deposition.
  • Contact dermatitis (Type IV): Redness, swelling, and itching at the site of contact allergen exposure.
  • Tuberculin skin test reaction (Type IV): Induration at the site of intradermal injection of purified protein derivative (PPD) in individuals previously exposed to Mycobacterium tuberculosis.

Diagnostic Approaches

Diagnosing hypersensitivity reactions involves:

  • Clinical evaluation: Assessing patient history, physical examination, and characteristic clinical features.
  • Laboratory tests: Including serologic tests, complement levels, and specific antibody titers depending on the suspected type of hypersensitivity.
  • Skin patch testing: Used to identify contact allergens in contact dermatitis.


Managing hypersensitivity reactions includes:

  • Avoid
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