Allergy (Drug Hypersensitivity)

Allergy (Drug Hypersensitivity) is an abnormal immune reaction to a drug, causing effects like rash, swelling, or anaphylaxis.

Definition of Drug Hypersensitivity:

• A drug allergy is an immune system-mediated adverse drug reaction.
• It can occur at low doses, is unpredictable, and is not related to the pharmacologic action of the drug.

Types of Hypersensitivity Reactions (Gell and Coombs classification):

1. Type I (Immediate, IgE-mediated):

   • Rapid onset (within minutes).
   • Examples: Anaphylaxis, urticaria.
   • Drugs: Penicillin, vaccines.

2. Type II (Cytotoxic, IgG/IgM-mediated):

   • Affects blood cells.
   • Example: Hemolytic anemia.

3. Type III (Immune complex-mediated):

   • Causes vasculitis, serum sickness.
   • Occurs after 1-3 weeks.

4. Type IV (Delayed, T-cell mediated):

   • Occurs after 48–72 hours.
   • Example: Contact dermatitis from topical drugs.

Management:

• Avoidance of the offending drug.
• Antihistamines, corticosteroids, or epinephrine (in case of anaphylaxis).

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