Urticaria is characterised by a particular form of lesion known as the wheal, which is a round, elongate or irregularly shaped, flat elevation of the skin due to a circumscribed tense edema of the dermis. Wheals vary in size, some maybe only as large as the head of a pin, others larger than the palm of the hand. To the touch they are moderately firm and cannot be made to disappear or pit on pressure. The color is pale pink or white, except at the margin where there is a rosy areola. The wheals appear suddenly and disappear more gradually, persisting from a few minutes to several hours. They are usually accompanied by lively sensations of itching, tingling and burning. When the response to mechanical stimulation is very pronounced, as above, the condition is often described as urticaria factitia or dermographia.