skin disorders
Hives
Causes of Hives
Hives and angioedema can be Overview of Allergic Reactions . Hives may occur when certain chemicals are inhaled, consumed, injected, or touched. These chemicals can be in the environment, foods, drugs (including medications), insects, plants, or other sources. They are harmless in most people. But if people are sensitive to them, these chemicals (called triggers or allergens) can cause an allergic reaction. That is, the immune system overreacts to the chemicals. However, most of the time, hives are not part of an allergic reaction and an allergen (the cause of the allergic reaction) cannot be identified. For example, they may result from Autoimmune Disorders . In these disorders, the immune system malfunctions, misinterpreting the body's own tissues as foreign and attacking them. Also, some drugs cause hives directly without triggering an allergic reaction. Some physical stimuli (such as heat, cold, pressure, friction, or sunlight) may cause hives for reasons that are not well understood. Hives usually last less than 6 weeks and are classified as acute. If hives last more than 6 weeks, they are classified as chronic. If a cause can be identified, acute hives are most commonly caused by Allergic reactions (such as foods and food additives, drugs, or insect bites) Nonallergic reactions (such as drugs, physical stimuli, or autoimmune disorders) Allergic reactions are often triggered by foods, particularly eggs, fish, shellfish, nuts, and fruits; food additives; drugs; or insect bites or stings. Eating even a tiny amount of some foods can suddenly cause hives. But with other foods (such as strawberries), an allergic reaction occurs only after a larger amount is eaten. Many medications, particularly antibiotics, may cause hives. Immediate allergic reactions may also occur when a substance comes into direct contact with the skin (such as latex), after an insect bite or sting, or as a reaction to a substance that is inhaled into the lungs or through the nose. Nonallergic causes of hives include infections, some drugs, and some physical stimuli (such as pressure or cold). In more than half of cases, a specific cause of acute hives cannot be identified. Possible identifiable causes of chronic hives are the same as those of acute hives. However, in the vast majority of cases, a cause cannot be identified (is idiopathic). Most cases of chronic hives that have no identifiable cause are thought to be due to an autoimmune reaction that itself has no detectable cause. Nevertheless, all efforts should be made to identify a cause, because elimination of a cause is the best approach to treatment. Sometimes the cause is easily overlooked, as when people repeatedly consume a food not known to be a trigger, such as a preservative or dye in foods or penicillin in milk. Often, despite the best efforts, the cause remains unidentified. Chronic hives can last for months or years, then sometimes go away for no apparent reason. 