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)