USMLE Guide: Immunology of Hypersensitivity Reactions

Introduction

This USMLE guide provides an overview of the immunology behind hypersensitivity reactions, which are exaggerated immune responses against harmless antigens. Understanding the underlying mechanisms and different types of hypersensitivity reactions is crucial for medical professionals, as they play a significant role in various diseases and clinical scenarios.

Table of Contents

1. Hypersensitivity Reactions: Overview
2. Types of Hypersensitivity Reactions
   • Type I: Immediate Hypersensitivity
   • Type II: Antibody-Mediated Hypersensitivity
   • Type III: Immune Complex-Mediated Hypersensitivity
   • Type IV: Delayed-Type Hypersensitivity
3. Pathophysiology of Hypersensitivity Reactions
4. Clinical Manifestations and Examples
5. Diagnosis and Treatment
6. Key Points to Remember

1. Hypersensitivity Reactions: Overview

Hypersensitivity reactions are immune responses that occur in individuals who have been sensitized to a specific antigen. These reactions are characterized by excessive immune activity that leads to tissue damage and inflammation. hypersensitivity reactions are classified into four different types (Type I, II, III, and IV) based on the underlying immune mechanisms.

2. Types of Hypersensitivity Reactions

Type I: Immediate Hypersensitivity

• Also known as allergic or anaphylactic reactions.
• Mediated by IgE antibodies.
• Examples include allergic rhinitis, asthma, anaphylaxis.
• The reaction occurs within minutes after exposure to the antigen.

Type II: Antibody-Mediated Hypersensitivity

• Mediated by IgG or IgM antibodies.
• Involves cell destruction through complement activation or antibody-dependent cell-mediated cytotoxicity (ADCC).
• Examples include autoimmune hemolytic anemia, transfusion reactions.
• The reaction occurs within hours to days after exposure to the antigen.

Type III: Immune Complex-Mediated Hypersensitivity

• Mediated by immune complexes formed by antigen-antibody interactions.
• Deposition of immune complexes leads to inflammation and tissue damage.
• Examples include systemic lupus erythematosus, post-streptococcal glomerulonephritis.
• The reaction occurs within hours to days after exposure to the antigen.

Type IV: Delayed-Type Hypersensitivity

• Mediated by T cells (CD4+ or CD8+).
• Involves recruitment and activation of macrophages and other immune cells.
• Examples include contact dermatitis, tuberculin skin test.
• The reaction occurs within 48-72 hours after exposure to the antigen.

3. Pathophysiology of Hypersensitivity Reactions

• Initial exposure to an antigen triggers the immune response, leading to sensitization.
• Upon re-exposure, the immune system mounts an exaggerated response.
• Different hypersensitivity reactions involve various immune cells, antibodies, and complement proteins.
• The release of inflammatory mediators (e.g., histamine, cytokines) contributes to tissue damage and clinical manifestations.

4. Clinical Manifestations and Examples

• Type I: Symptoms range from mild (rhinitis, urticaria) to severe (bronchospasm, anaphylaxis).
• Type II: Manifestations depend on the target tissue or organ affected (e.g., hemolysis, thrombocytopenia).
• Type III: Presents as immune complex-mediated diseases affecting multiple organs (e.g., systemic lupus erythematosus).
• Type IV: Results in delayed inflammatory reactions, such as skin rashes or granulomas.

5. Diagnosis and Treatment

• Diagnosis is based on clinical presentation, history, and specific diagnostic tests (e.g., skin prick tests, laboratory tests).
• Management includes avoiding triggers, pharmacotherapy (e.g., antihistamines, corticosteroids), and desensitization (for Type I).
• Severe reactions may require emergency interventions like epinephrine administration and airway support.

6. Key Points to Remember

• Hypersensitivity reactions are exaggerated immune responses against harmless antigens.
• Four types of hypersensitivity reactions exist: Type I (IgE-mediated), Type II (antibody-mediated), Type III (immune complex-mediated), and Type IV (delayed-type).
• Each type involves different immune mechanisms and exhibits unique clinical manifestations.
• Diagnosis is based on history, clinical examination, and specific tests.
• Treatment involves avoiding triggers, symptomatic management, and emergency interventions if necessary.

Remember to thoroughly study the underlying immunological mechanisms, clinical presentations, and management of each hypersensitivity reaction type for the USMLE exam.