Understanding Hypersensitivity Reaction Types

Sep 7, 2024

Hypersensitivity Types

Hypersensitivity reactions are classified into four types, each with unique mechanisms and examples. Understanding these is essential for exams like USMLE and COMLEX.

Mnemonic: ACID

  • Type 1: Allergic
  • Type 2: Cytotoxic
  • Type 3: Immune Complex
  • Type 4: Delayed

Type 1: Allergic Hypersensitivity

  • Mechanism: Antigen binds to IgE → Cross-linking at FC receptor → Mast cells and basophils release vasoactive substances → Vasodilation.
  • Examples: Allergy, Asthma, Anaphylaxis
  • Key Points:
    • Cross-linking involves IgE
    • Massive vasodilation leads to allergic reactions
    • Mnemonic: A for Allergic, Asthma, Anaphylaxis

Type 2: Cytotoxic Hypersensitivity

  • Mechanism: IgG or IgM binds to antigen → Complement activation → Possible natural killer cell activation → Cell death
  • Examples: Goodpasture’s syndrome
  • Key Points:
    • Involves IgG or IgM
    • Results in death of self cells
    • Smooth (linear) antibody deposition
    • Mnemonic: C for Cytotoxic

Type 3: Immune Complex Hypersensitivity

  • Mechanism: IgG or IgM forms complexes with antigens → Deposition in tissues, usually basement membrane → Complement activation → Tissue damage.
  • Examples: Post-streptococcal glomerulonephritis
  • Key Points:
    • Complexes are lumpy, bumpy
    • Contrast with Type 2’s smooth deposition
    • Mnemonic: I for Immune Complex

Type 4: Delayed Hypersensitivity

  • Mechanism: Antigen sensitizes TH1 or CD8+ cells → Cytokine release → Macrophage activation → Target cell removal
  • Examples: Contact dermatitis, TB test reaction
  • Key Points:
    • Delayed reaction (e.g., TB test takes 48 hours to read)
    • Mnemonic: D for Delayed, Dermatitis, Diabetes, Diagnosing TB

High-Yield Considerations

  • Type 2 vs. Type 3:
    • Goodpasture’s Syndrome (Type 2): Smooth, linear deposition
    • Post-streptococcal Glomerulonephritis (Type 3): Lumpy, bumpy deposition
    • Both involve antibodies but differ in deposition patterns

Summary

  • Mnemonic ACID helps remember the types.
  • Emphasis on deposition patterns and clinical examples.
  • Understanding these differences is crucial for exams.

These notes simplify a complex topic and highlight key points critical for academic success in exams.