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Nettle rash is a condition that is due to an immune system problem, and not usually from exposure to nettles. The rash appears on the skin as red areas that may be raised in the skin. Although the condition can resolve on its own over the course of a day, it does tend to return over and over to the affected person. Possible causes include allergies to food, reactions to medicine and exposure to animals.
In a healthy person, the immune system protects the body from dangers like infectious disease. Sometimes the inflammation, redness and swelling that the immune system can use as tools against infection is produced accidentally in response to substances that should not pose any significant danger to the body. This occurs in cases of nettle rash, but often, the person cannot figure out what substances, or conditions, cause the rash.
Typically, a nettle rash produces raised hives on the skin that are red and itchy. This condition is also called either urticaria or hives, and may be present in a chronic form affecting people regularly, or an acute form, happening only rarely. Although the locations of the spots on the skin can change over time, a typical case goes away after only one day.
The swelling and the inflammatory redness is caused by the presence of a substance called histamine in the affected area. Histamine levels rise with allergic immune response, and signal to the blood vessels in the vicinity of the supposed invader to leak fluid into the skin, thus making the raised bumps associated with the condition. As histamine is an integral part of the development of hives, an antihistamine medication can be beneficial to the affected person.
When the person is able to figure out what the cause of the rash is, he or she may find it is a natural, artificial or a physical cause. Natural triggers include pollen, insect bites and foodstuffs. Artificial causes include medications, such as antibiotics and over-the-counter anti-inflammatories. Ingredients in perfume or nickel exposure can also produce the characteristic rash. Even alterations in the physical environment can produce nettle rash, such as exposure to the sun, physical pressure on the skin, or changes in body temperature.
If a person experiences many episodes of the rash, he or she may also have an underlying immune system problem, but this is not common. Some cases of nettle rash, characterized by rash around the mouth, on the hands and next to the eyes, are caused by a condition called hereditary angioneurotic oedema. This is caused by genetic mutations, and requires a doctor's advice rather than antihistamine treatment.