User Tools

Site Tools

Trace: • propranolol • hypersensitivity_types
allergy:immunology:hypersensitivity_types

Hypersensitivity Reactions (Types I–IV)

Hypersensitivity reactions are exaggerated or misdirected immune responses that result in tissue injury.

Classified by mechanism:

  • Type I – IgE-mediated (Immediate)
  • Type II – Antibody-mediated cytotoxic
  • Type III – Immune complex deposition
  • Type IV – T-cell mediated (Delayed)

Type I – Immediate (IgE-Mediated)

Mechanism:

  • Allergen → IgE production
  • IgE binds mast cells
  • Re-exposure → mast cell degranulation

Onset:

  • Minutes

Examples:

Primary mediators:

  • Histamine
  • Leukotrienes
  • Prostaglandins

See:

Type II – Antibody-Mediated Cytotoxic

Mechanism:

  • IgG or IgM binds to cell surface antigen
  • Complement activation
  • Cell destruction

Onset:

  • Hours to days

Examples:

  • Autoimmune hemolytic anemia
  • Immune thrombocytopenia (ITP)
  • Goodpasture syndrome
  • Certain drug-induced cytopenias

Key concept: 

Antibody directed against fixed tissue antigen.

Type III – Immune Complex Disease

Mechanism:

  • Antigen–antibody complexes form in circulation
  • Deposit in tissues
  • Activate complement
  • Inflammation and tissue injury

Onset:

  • Days to weeks

Examples:

  • Serum sickness
  • Systemic lupus erythematosus (SLE)
  • Post-infectious glomerulonephritis

Key concept: 

Injury from immune complexes, not direct antibody binding.

Type IV – Delayed (T-Cell Mediated)

Mechanism:

  • Sensitized T cells recognize antigen
  • Cytokine release
  • Cytotoxic T-cell–mediated damage

Onset:

  • 48–72 hours or longer

Examples:

  • Contact dermatitis (poison ivy)
  • Tuberculin skin test
  • Stevens-Johnson Syndrome (SJS)
  • Toxic Epidermal Necrolysis (TEN)
  • DRESS syndrome

Key concept: 

No antibodies involved.

Quick Comparison Table

Type Immune Driver Antibodies? Onset Example
I IgE + Mast Cells Yes (IgE) Minutes Anaphylaxis
II IgG/IgM vs cell surface Yes Hours–Days Hemolytic anemia
III Immune complexes Yes Days–Weeks Serum sickness
IV T cells No 48–72 hr Contact dermatitis

Drug Implications

Type I:

Type II:

  • Immunosuppressants
  • Steroids
  • Plasma exchange (select cases)

Type III:

  • Steroids
  • Immunomodulators

Type IV:

  • Corticosteroids
  • Drug discontinuation
  • Immunosuppressive therapy (severe cases)

High-Yield Differentiation

If reaction occurs within minutes → think Type I.

If reaction causes cytopenias → think Type II.

If rash + arthralgias + immune complexes → think Type III.

If delayed blistering rash after drug exposure → think Type IV.

allergy/immunology/hypersensitivity_types.txt · Last modified: by andrew2393cns