Anaphylactic reaction vs anaphylactoid reaction

Anaphylactic reaction

• Exaggerated response to a foreign substance
• Previous exposure/sensitisation necessary
• Subsequent exposure triggers massive degranulation by mast cells, mediated by specific IgE antibodies
• Response is not related to quantity of the triggering allergen

Anaphylactoid reaction

• Triggered by
  * Direct stimulation on mast cells, causing histamine release
  * Complement activation (classical or alternative pathways)
• No prior exposure necessary
• No IgE antibody involvement

Example of anaphylactoid reaction

• Common in reactions to contrast media

Non-immunological histamine release

Drug directly acting on mast cells
--> release of histamine

• Response is related to quantity of the triggering allergen
• Tend to be confined to skin
• Occurs in up to 30% of anaesthesia

Examples of drugs directly stimulating mast cells

• Atracurium
• Mivacurium
• Morphine
• Meperidine