URTICARIA, popularly known as nettlerash, is characterised by the appearance on the skin of slightly raised red or white patches, varying in shape and size, termed wheals, which develop suddenly, persist for a few hours, and then fade spontaneously and completely. The eruption is invariably accompanied by sensations of itching or burning which may be very pronounced. It is cus tomary to distinguish between an acute and a chronic variety of urticaria. The former is marked by its abrupt onset, the profuse and extensive character of the eruption, and in many cases by the general symptoms which accompany the rash, such as fever, diarrhoea and vomiting. Acute urticaria is often caused by tainted food, and the whole attack is usually limited to a few hours or a few days. In chronic urticaria the eruption is either continuously present over periods of months or years, or is developed in bouts with intervals of comparative or absolute freedom. The onset as a rule is insidious, and the general health is little disturbed unless the degree of itching is sufficient to interfere with sleep or repose.
Nettlerash is now generally held to represent a symptom rather than a genuine disease, for it may be provoked by a great many and very different agents acting externally (nettles, jellyfish), or internally. The theory of sensitisation has a special application
in urticaria. For example different kinds of foods are capable of causing urticaria under special conditions. A certain individual suddenly becomes unduly "sensitive" to some article of diet which has, up to this point, been eaten without harm, while a second individual develops "hypersensitiveness" to a completely different type of food, a third to something different again, and so on. This acquired condition is specific, that is to say the individual who is "sensitised" only reacts to what is now his own harmful sub stance (see ANAPHYLAXIS).
The human tissues can also become "sensitised" to many other agents besides food, such as the poisons liberated from collec tions of germs in the teeth, tonsils, bowel and other regions of the body (focal infection). In other cases a second injection of horse-serum (diphtheria or tetanus anti-toxin), if given at a suffi cient interval of time after the first will sometimes cause urticaria and anaphylactic shock. The mechanism of the reaction is similar in all these cases and the same degree of specificity is observed.
The theory of sensitisation does nit cover every case as, for example, where the eruption is related to an emotional crisis.
(H. MAcC.)