Given that this text refers to the issue of allergy, I recommend reading it along with a more general text about allergy that describes the issues of atopy, allergens and allergic reactions.
Urticaria or hives is an allergic skin disease that affected sometimes in their life the majority of population but in some people, it recurs frequently.
The cause of urticaria is a local allergic reaction to an allergen, which is limited to the skin and subcutaneous tissue. Allergic reaction leads to leakage of a relatively aggressive substance known as histamine from cells to the surrounding area. This process causes local irritation and skin inflammation with increased permeability of blood vessels for blood plasma. There are many possible allergens that can cause hives including food (food allergy), medications and in many people even a change of the ambient temperature.
As the local irritation leads to increased permeability of blood vessels, blood fluid gets out of the vessels into the subcutaneous tissue and causes local swelling. The histamine is an irritant causing itching giving the lesions form of itchy reddish skin buds. Itching may be followed by scratching, which further damages the skin. Luckily, urticaria usually quickly disappears. The hives may be also a part of a serious allergic condition known as the anaphylactic shock.
The diagnosis can be easily stated from the clinical symptoms. Blood tests may confirm elevated blood levels of IgE antibodies (responsible for acute allergic reactions). In addition, it is advisable to find the inducing factor. Further tests are generally not necessary with the exception of severe and frequently recurring forms of the hives.
The best prevention is to find the inducing factor and try to avoid it.
Urticaria is usually self-limiting and quickly disappears without any treatment. Prolonged and unpleasant manifestation can be treated by some anti-allergic medications, especially by antihistaminic agents. It is advisable not to scratch the affected skin areas as it can damage the skin cover and lead to local bacterial infection. When the urticaria is related to other symptoms of allergy, more powerful medications can be administered for a short period such as the corticosteroids. Anaphylactic shock must be treated during hospitalization by large doses of corticosteroids and prevention of a shock condition development.