Urticaria
Mast-cell degranulation releases histamine into the superficial dermis, causing local vasodilation and fluid leak that produce short-lived, itchy, blanching weals: the same mechanism that, in deeper tissue or on a larger scale, becomes angioedema or anaphylaxis.
First principles
Urticaria is histamine released into the skin, nothing more mysterious
The central event in urticaria is mast cell degranulation. This can be triggered by an IgE-mediated allergic reaction, direct mast-cell stimulants (certain drugs and foods), physical stimuli such as pressure, cold or heat, or, in chronic spontaneous urticaria, autoantibodies against the mast cell's own IgE receptor. Whatever the trigger, the final common pathway is the same: histamine and other mediators are released into the superficial dermis.
Educational content pending clinical review. Not medical advice.