Urticaria and angioedema

Urticaria (hives) is a common eruption characterized by transient, usually pruritic, wheals due to acute dermal oedema from extravascular leakage of plasma. Angioedema signifies a larger area of oedema involving the dermis and subcutis. A classification is shown in Table 1.

Table 1 Classification of urticaria and angioedema

Allergic (IgE mediated) mast cell degranulation	SystemicSkin contact	Food, drugs, latex (aerosols)Animal saliva, pollen, latex
Non-allergic (non-IgE mediated) mast cell degranulation	Chronic ordinaryPhysicalPharmacological	No cause identifiable (commonest subtype)Dermographism, cholinergic, cold, solar, heat, delayed pressureAspirin, opiates, non-steroidal drugs, food additives, ACE inhibitors (p. 87)
	Autoimmune disease	Systemic lupus erythematosus (p. 80), thyroid antibodies, anti-IgE receptor antibodies, urticarial vasculitis (p. 77)
	Genetic	C1 esterase inhibitor deficiency (p. 77), mastocytosis (p. 116)
	Other	Infection, paraneoplastic, skin contact (nettle sting)

Aetiopathogenesis

Urticaria is mediated through immune (allergic) or non-immune mechanisms. Lesions result from the release from mast cells of biologically active substances, particularly histamine, which produce vasodilatation and increased vascular permeability. Several pathways are recognized:

IgE-mediated (type I) hypersensitivity (p. 11) is the best understood mechanism; antigen cross-links immunoglobulin (Ig) E molecules on the surface of mast cells, resulting in degranulation with release of vasoactive agents.