Urticaria is classified according to (1) its duration and (2) its triggers of exacerbation (Table 23.1). In acute urticaria , the signs and symptoms occur for less than 6 weeks, whereas chronic urticaria is of more than 6 weeks duration [42]. The development of wheals and angioedema in urticaria patients can either be unpredictable and unprompted, in spontaneous urticaria, or it can occur only in response to specific triggers and eliciting situations, in inducible urticarias. In the inducible urticarias, specific triggers, which can be exogenous and acting on the skin directly (physical urticarias, contact urticaria, aquagenic urticaria) or not (cholinergic urticaria), are responsible and required for the induction of signs and symptoms. Triggers of physical urticaria are skin contact with cold and heat (cold urticaria, heat urticaria), mechanical triggers such as friction, pressure and vibration (symptomatic dermographism, pressure urticaria, vibratory urticaria, respectively), or ultraviolet or visible light (solar urticaria) [42]. Triggers of symptoms in contact urticaria are skin contact with urticariogenic substances, and in the case of aquagenic urticaria exposure to water. In cholinergic urticaria, symptoms are brought about by exercise or passive warming (sauna, hot showers, spicy food). Frequently, patients show more than one urticaria type, for example chronic spontaneous urticaria and symptomatic dermographism.

Urticaria is a very common disease and usually of short duration (acute urticaria). Virtually everyone, at one point during his or her life, develops acute urticaria. Most commonly, this is acute contact urticaria, caused by skin contact with urticariogenic substances derived, for example, from stinging nettles or jellyfish (Fig. 23.3). Acute spontaneous urticaria is also very common, and it is estimated that its lifetime prevalence is up to 20 % [25]. Acute spontaneous urticaria rarely progresses to chronic spontaneous urticaria. Nonetheless, chronic spontaneous urticaria, which is thought to be twice as frequent as chronic inducible urticaria, is estimated to have a point prevalence of 0.5–1 % [25]. Chronic spontaneous urticaria can affect all age groups with a peak incidence between the twentieth and the fortieth year of life; that is, patients are primarily affected during important years of their working age [25]. Women are affected about twice as often as men [25].

The signs and symptoms of urticaria, both spontaneous and inducible, are due to the activation and degranulation of skin mast cells and the effects of pro-inflammatory mediators released in the process. Mast cells are long-lived resident cutaneous cells that predominantly localize around blood vessels and sensory nerves [26] as well as in the upper papillary dermis. They contain cytoplasmic granules in which preformed mediators are stored that are released into the cell’s vicinity by degranulation in response to activation [36]. These preformed mast cell mediators include histamine , heparin, and proteases (e.g., tryptase, chymase) [7] as well as several cytokines. Upon activation and degranulation, skin mast cells also rapidly produce and secrete prostaglandins, leukotrienes, and platelet activating factor [36]. Mast cells are thought to function as sentinel cells of the skin and a first line of defense against bacteria and other pathogens [27]. The inflammatory effects of their mediators include the activation of sensory skin nerves (itch, burning sensation, pain), the dilatation of skin blood vessels (erythema, hyperthermia) and the induction of plasma extravasation (wheals and angioedema). Histamine plays a critical role in skin mast cell-mediated vasodilation and extravasation, by acting on H1 receptors [8]. The degranulation of skin mast cells in urticaria patients also results in the recruitment of basophils, neutrophils, eosinophils, and other immune cells to the site of activation [41]. The mechanisms of mast cell activation in urticaria are largely unclear. In chronic urticarias, mast cells are typically not activated by the binding of environmental allergens to specific IgE bound to cell surface IgE receptors [42], as is the case in allergies such as allergic rhinitis and anaphylaxis. Reported candidates for relevant signals involved in mast cell activation in chronic spontaneous urticaria include autoantibodies to IgE [11] or the IgE receptor [15], IgE autoantibodies directed against autoantigens (autoallergens) such as thyreoperoxidase [2], complement components such as C5a [10], as well as neuropeptides [4], for example, substance P.