Location and duration:
Well, urticaria may appear on any part of the skin. Angioedema is a condition when deep tissues are affected. The typical lesions may last for one minute to half an hour. Some may last even longer. Some patients may get the eruptions once in a while and some may have many times during the day. It may be restricted to a couple of spots in some patients, while some may have wide spread rashes appearing for days or even months together.

There are acute, subacute, chronic and recurring variants as far as the frequency and duration are concerned.

Histopathological view:
Under the microscope, a typical urticarial rash may exhibit perivascular, cellular infiltrate consisting of lymphocytes and eosinophils, is indicative of its allergic behavior. There are findings related to oedema (swelling) and mucosal inflammation.

The Inner War:
The urticaria rash is a symptom of an allergic and immunological event taking place at the dermal level. The exact understanding is illusive to an extent. In brief, urticaria is a hypersensitive reaction due to the histamine release. The histamine release could be from the mast cells when antigens and antibodies (IgM or IgG) combine to activate the immunological reaction. The histamine release could IgE induced. There are certain drugs, pharmacological agents (e.g.: antibiotics, morphine, aspirin, etc.), food articles (proteins, milk products, etc.) Urticaria is a sign of antigen-antibody reaction.
During this process of antibody-antigen reaction, histamine and/or acetyl choline is generated which has the property of causing vessel dilatation (vasodilation) swelling, itching, pain and rash.