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:
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