BACKGROUND

Local wheals (synonyms: “nettle rash,” hives) and erythema that resemble the effect of the common stinging nettle (Urtica dioica) on the skin is known as urticaria. The edema involves superficial skin to the mid-dermis. Pruritus is common.

Angioedema (synonyms: angioneurotic edema, Quinke's edema) produces a similar eruption but with larger edematous areas that affect both the dermal and subcutaneous and/or submucosal tissues. Angioedema is usually painful rather than itchy and is less well defined or normal in color. Both types of reaction can be triggered by drug allergies, insect stings or bites, desensitisation injections, cold temperature, other physical stimuli, or ingestion of certain foods, particularly eggs, shellfish, nuts, or fruits.

Mastocytosis is caused by an abnormal conglomeration of mast cells at a particular site. Degranulation, through rubbing or contact, triggers release of excessive histamine, resulting in localized swelling.

Anaphylaxis represents an extreme form of acute allergic reactions that is mediated by immunoglobulin E (IgE). Anaphylaxis is an example of a type I hypersensitivity reaction (see Chapter 19). Although erythema, urticaria, and angioedema may all occur, it is the systemic hypotension and shock that determine the outcome. It is a clinical emergency characterized by profound shock that may rapidly lead to cardiorespiratory arrest. A similar picture from nonallergic causes is called an “anaphylactoid reaction.”