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Hives (urticaria) may be caused by allergic or nonallergic reactions. Allergic urticaria occurs when the body misinterprets a substance (allergen) as being a threat and then attacks that threat (allergic reaction). It is the less common form of hives, and it is seen more often in children than adults.

When a definitive allergic basis cannot be found or proven, hives are classified as nonallergic. Exposure to extreme temperatures, repeated stroking of the skin, psychological stress, reaction to diseases and frequent bacterial and viral infections are examples of nonallergic urticaria triggers. Women in the last trimester of pregnancy may also experience hives, which usually resolve after delivery.

Urticaria is often classified according to the cause and/or length of time the episodes last. Examples include:

• Acute urticaria. Episodes of hives that last for less than six weeks. Most episodes of allergic hives are acute and persist for a few days after exposure to the allergen.
  
  
• Aquagenic urticaria. Hives caused by contact with water.
  
  
• Cholinergic urticaria. Hives caused by exercise, passive body warming (shower or bath) or emotional stress. The wheals (individual hives) usually occur around the lips or larynx, although they can appear anywhere on the skin. These hives are generally the smallest type seen, appearing on the skin like goosebumps. This allergic reaction can also lead to lung symptoms, such as shortness of breath.
  
  
• Chronic idiopathic urticaria. Episodes of hives that persist for more than six weeks (chronic) and for which the cause is unknown (idiopathic). This form is seen most often in middle-aged women, but it can affect all ages, genders and races. Chronic, idiopathic urticaria can persist for years and has been traditionally believed to be nonallergic. However, recent research indicates that some patients develop an allergy to components in their own skin, continuously triggering hives. Researchers hope their findings will lead to a better understanding of the causes of chronic idiopathic urticaria.
  
  
• Cold urticaria. Hives caused by cold temperatures.
  
  
• Contact urticaria. A form of allergic urticaria caused when the skin comes in direct contact with an allergen (e.g., in a person with a latex allergy, it may be caused by wearing latex gloves).
  
  
• Pressure urticaria. Hives caused by pressure on the skin. There are two main forms:
  
  
• • Dermatographism involves wheals that form in areas where clothing applies friction to the skin.
    
    
  • Delayed pressure urticaria involves wheals that form under belts, shoulder straps or bras and are more diffuse in appearance.
    
    
• Solar urticaria. Hives caused by direct exposure to the sun. Wheals may appear within minutes of sun exposure (photosensitivity).
  
  
• Vasculitic urticaria (urticarial vasculitis). Hives that stay in the same place for more than a day and that leave a pigmented bruise mark when the swelling has resolved. Vasculitic urticaria may be part of an illness, such as rheumatoid arthritis, which also affects other parts of the body.
  
  
• Exercise-induced anaphylaxis. A related condition, which may be triggered by a combination of physical exertion and the eating of certain foods. It is differentiated from cholinergic urticaria by the structure of the lesions and the degree of response to passive body warming.
  

These different types of hives often coexist, further complicating a diagnosis. For instance, cold urticaria and pressure urticaria often occur together.