Hypersensitivity Types Overview

Hypersensitivity can be divided into four types, each with distinct mechanisms and examples, significant for exams like USMLE and COMLEX.

Mnemonic: ACID

• A: Type 1
• C: Type 2
• I: Type 3
• D: Type 4

Type 1 Hypersensitivity: Allergic

• Mnemonic: A for Allergic, Asthma, Anaphylaxis, Allergy
• Mechanism:
  • Antigen binds to IgE.
  • Cross-linking occurs at the Fc receptor (not needed for exams but relevant for class exams).
  • Mast cells and basophils release vasoactive substances, causing vasodilation.
  • Example: Anaphylaxis, countered by epinephrine (causes vasoconstriction).
• Key Terms: IgE, mast cells, basophils, vasodilation.

Type 2 Hypersensitivity: Cytotoxic

• Mnemonic: C for Cytotoxic
• Mechanism:
  • IgG or IgM bind to an antigen on a cell receptor.
  • Complement activation occurs, possibly natural killer cell activation.
  • Results in the death of native cells.
  • Antibody deposition is smooth.
• Examples: Goodpasture syndrome (smooth deposition).
• Key Terms: IgG, IgM, complement activation, cytotoxic, smooth.

Type 3 Hypersensitivity: Immune Complex

• Mnemonic: I for Immune Complex
• Mechanism:
  • IgG or IgM bind to antigens forming immune complexes.
  • Complexes deposit in tissues, activating complement and causing tissue damage.
  • Complex deposition is lumpy bumpy or globular.
• Examples: Poststreptococcal glomerulonephritis (lumpy bumpy deposition).
• Key Concepts:
  • Compare Goodpasture syndrome (type 2, smooth) vs. Poststreptococcal glomerulonephritis (type 3, lumpy bumpy).
  • Linear vs. lumpy bumpy deposition is high yield for exams.
• Key Terms: Immune complex, complement activation, lumpy bumpy.

Type 4 Hypersensitivity: Delayed

• Mnemonic: D for Delayed
• Mechanism:
  • Antigen sensitizes a Th1 CD8+ cell.
  • Cell releases cytokines, activating macrophages.
  • Direct removal of target cells by macrophages.
  • Delayed response (e.g., TB test requires 48 hours for a reaction).
• Examples: Type 1 diabetes, contact dermatitis, tuberculosis diagnosis.
• Key Terms: Th1 cells, CD8+ cells, cytokines, macrophages, delayed response.

Summary

• Mnemonic Recap:
  • A: Allergy
  • C: Cytotoxic
  • I: Immune Complex
  • D: Delayed
• Key High-Yield Point:
  • Differentiate smooth (Goodpasture, type 2) vs. lumpy bumpy (Poststreptococcal, type 3) deposition.
• Clinical Relevance:
  • Recognizing and differentiating between types is crucial for exams. Understanding pathology and clinical examples aids in application.

Focus on understanding deposition patterns and their clinical implications for exam preparation. Good luck!