What happens in the skin during urticaria?
Trigger. Many substances can trigger itchiness, whether applied to the skin externally, ingested or inhaled, or produced by the body itself. One common characteristic for most of these substances is that they release histamine, a neurotransmitter believed to play a key role in the triggering of itchiness. This itch-triggering effect of histamine is very clearly evident after an insect bite, for example, or after contact with stinging nettles. Beyond substances that release the body's supply of histamine, the venom of many insects and pruritic (itch-inducing) plants contains histamine that penetrates and stimulates the skin. The impulse is perceived by specialised nerves (the "itch nerves," if you will). A message is sent to the brain, the central switchboard for the nerves: something's itching here! This isn't always a bad thing at all. For insect bites or following contact with pruritic plants, itchiness induces us to scratch the skin, cool it under cold water (and thereby remove the trigger) or rub it (bringing more blood to the spot of the reaction, allowing the itch stimulant or venom to be more quickly transported away through the blood stream). Almost all of the skin's histamine is stored in a specific kind of cell, known as mast cells. If these cells are activated, i.e. if they are "nettled" by a stimulus (and there are a variety of stimuli that can do this), then the mast cells release their histamine into the surrounding tissues, leading to wheal formation and itching.