What causes or triggers hives?

Hives may be caused or triggered by the following:

Allergies

Most often, an allergic reaction causes an outbreak of hives. Once an allergen has been encountered (been made contact with) or swallowed (ingested), the body responds by releasing histamines into the bloodstream. ⁽²⁾ Histamine is essentially an organic nitrogenous compound which affects immune responses (i.e. histamines are released as a defence mechanism against infection or what is determined to be an ‘intruder’ in the body). Histamine also functions as a neurotransmitter (chemical messenger) for the spinal cord, uterus and brain.

Allergic and inflammatory reactions in the body can influence histamine levels in the gastrointestinal tract, which go on to cause an imbalance between histamine accumulation and degradation, as well as have an impact on microbial processes and dietary intake. The result is a histamine intolerance / impaired enzymatic degradation. Intolerance of histamine has been associated with a variety of symptoms including hives, which mimic an allergic reaction. The condition is rare and only a very small percentage of people develop a histamine intolerance. The development of hives as result of an intolerance are also uncommon. 

In an allergic reaction, histamines can aggravate the body by leaking out of small blood vessels (capillaries) and into the skin, resulting in inflammation (swelling), itching and stinging sensations due to an accumulation of this fluid.

This reaction may be triggered by elements such as:

• Pollen
• Foodstuffs (or chemicals in certain foods, food colouring and preservatives) – fish, shellfish, eggs, milk, strawberries, wheat, gluten, tree nuts and peanuts
• Animal dander – dogs, cats or horses
• Dust mites
• Plants – poison ivy or poison oak, and nettles
• Medications (antibiotics and nonsteroidal anti-inflammatories / NSAIDS)– most common are those containing penicillin, aspirin, naproxen, ibuprofen and some blood pressure drugs (ACE / angiotensin-converting-enzyme inhibitors)
• Insect bites or stings
• Cockroaches and cockroach waste
• Latex
• Chemicals – fragrances, cleaning detergents, shampoos, hair dyes, cosmetics and bath or shower products
• Sunlight exposure - which leads to solar urticaria (hives due so sun exposure)
• Water (on the skin) - this can cause aquagenic urticaria (hives due to water exposure)

Other common hives triggers

Non-allergic triggers can include:

• Exercise and excessive sweating (resulting in body heat-inducing hives) ⁽³⁾
• Excessive exposure to hot or cold temperatures (air exposure) and high body temperature (cholinergic urticaria)
• Rubbing, scratching or friction with the skin (dermagraphism)
• Pressure due to constrictive clothing or in areas of the body under constant pressure (such as the soles of the feet)
• Stress (this is an emotional aggravator that has the potential to elicit a number of physical reactions in the body)
• A viral or bacterial infection – hepatitis, Epstein-Barr virus, HIV/AIDS, flu (influenza), and cytomegalovirus (belonging to the herpes family of viruses), urinary tract infection and strep throat
• Underlying medical conditions – lupus, lymphoma and thyroid disorders, as well as a response to blood transfusions and blood products
• Inflammation of the blood vessels (vasculitis) – these can be painful and itchy, as well as leave a bruise on the skin
• Intestinal parasites

References:

^{2. Medline Plus. August 2018. Hives: https://medlineplus.gov/ency/article/000845.htm [Accessed 29.08.2018]}

^{3. US National Library of Medicine - National Institutes of Health. August 2008. An approach to the patient with urticaria: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2492902/ [Accessed 29.08.2018]}