mechanisms

Last reviewed 01/2018

Mechanisms of urticaria

• mechanisms of urticaria can be either:
  • immune-mediated - IgE located on the mast cells an basophils cross-links with the allergen and activates the cells e.g.- urticaria induced by medication (penicillin), foods
  • complement-mediated - mast cells are activated directly by compliments ( mainly by C3a, C4a, and C5a) e.g.- serum sickness and transfusion reactions which cause the activation complement cascade leading to urticaria
  • non-immune mediated - mast cells are activated by non IgE mechanisms e.g.- physical stimuli, alcohol, radiocontrast dye, and medications (opiates, vancomycin, aspirin)
  • autoimmune-mediated - circulating auto-antibodies activates the mast cells (1)
• all of the above mentioned mechanisms lead to dermal mast cells degranulation in response to a variety of stimuli releasing mediators which produce vasodilatation, dermal oedema, and a perivascular infiltrate of lymphocytes and eosinophils (1). When deep dermis / subcutaneous tissues involved = angioedema
• vasoactive mediators commonly released during urticaria include:
  • histamine
  • prostaglandin D 2
  • leukotrienes C 4 and D 4
  • platelet activating factor
  • cytokines (1)

Reference:

• 1. American Academy of Emergency Medicine 2006. Clinical Practice Guideline: Initial Evaluation and Management of Patients Presenting with Acute Urticaria or Angioedema