What causes hives?
A trigger causes cells in the skin to release chemicals such as histamine. The chemicals cause fluid to leak from tiny blood vessels under the skin surface. The fluid pools to form weals. The chemicals also cause the blood vessels to open wide (dilate) which causes the flare around the weals. The trigger is not known or identified in many cases. Possible causes in some cases include the following:
- In many cases the cause may be an autoimmune problem. Autoimmune means that our own immune system causes damage to some of our body's own cells. Normally, our body makes proteins called antibodies to fight infections - for example, when we catch a cold or have a sore throat. These antibodies help to kill the germs causing the infection. In autoimmune diseases the body makes similar antibodies (autoantibodies) that attack its normal cells. In hives (urticaria), these antibodies attach to cells under the skin and cause them to release histamine and other chemicals. The reason why this happens is not clear.
- Stress, infections or medicines may be a trigger in some cases.
- An allergy to a food, medicine or parasite (such as worms in the gut) is an uncommon cause of hives. A skin specialist may advise tests if an allergy is suspected.
- Inducible urticaria - sometimes called physical urticaria. This is a type of hives in which a rash appears when the skin is physically stimulated. The most common example of this is called dermatographism (dermatographia) - a rash develops over areas of skin which are firmly stroked. In other cases an urticarial rash is caused by heat, cold, emotion, exercise, or strong sunlight. See separate leaflet called Hives (Inducible Urticaria) for more details. This kind of hives often causes bouts of sudden-onset (acute) symptoms, but sometimes causes chronic symptoms.
Types of hives
Hives (urticaria) can be classed as follows:
- Acute - if it develops suddenly and lasts less than six weeks. Most cases last 24-48 hours. About 1 in 6 people will have at least one bout of hives in their lives. It can affect anyone at any age. Some people have repeated bouts of hives. See separate leaflet called Hives (Acute Urticaria) for more details.
- Chronic - if it persists long-term. (Chronic means persistent or ongoing.) In the chronic condition, a rash develops on most days for at least six weeks. This is uncommon. About 1 in 1,000 people develop chronic urticaria at some stage in their lives. It is more common in women than it is in men. Some people have an urticarial rash on and off for months, or even years.
Did you find this information useful?
- BSACI guideline for the management of chronic urticaria and angioedema; British Society for Allergy and Clinical Immunology (Feb 2015)
- Urticaria; NICE CKS, May 2016 (UK access only)
- Zuberbier T, Aberer W, Asero R, et al; The EAACI/GA(2) LEN/EDF/WAO Guideline for the definition, classification, diagnosis, and management of urticaria: the 2013 revision and update. Allergy. 2014 Jul 69(7):868-87. doi: 10.1111/all.12313. Epub 2014 Apr 30.
- Urticaria; DermNet NZ
- Urticaria and angio-oedema: an overview; Primary Care Dermatology Society
- Omalizumab for previously treated chronic spontaneous urticaria; NICE Technology Appraisal Guidance, June 2015
- Sharma M, Bennett C, Cohen SN, et al; H1-antihistamines for chronic spontaneous urticaria. Cochrane Database Syst Rev. 2014 Nov 14 (11):CD006137. doi: 10.1002/14651858.CD006137.pub2.