Rashes come in many forms, from the raised, red patches of hives -- also known as urticaria -- to the itchy blisters of poison ivy. Some rashes are painful, some are painless, and others are accompanied by itching. The urge to scratch an itchy rash should be resisted, as doing so may introduce bacteria to the skin and cause infection. Various factors cause itching and rashes.
Immune Reactions
When the immune system detects that a person has come in contact with a substance that it deems harmful, it releases a protein called histamine, according to KidsHealth. Histamine increases the permeability of blood vessel walls. Fluids that carry white blood cells then flow through the walls, out of the blood vessels and into the affected tissue. In cases where itchy rashes are involved, the inflammation that this increased fluid causes manifests in symptoms affecting the skin. Allergic reactions occur when the immune system misidentifies a typically harmless substance as a pathogen. An allergic reaction to a substance taken internally, such as a medication, or a substance that the skin has contacted, such as poison ivy, can result in a skin rash.
Infections
Viruses may cause itching and rash long after the initial infection has passed. For example, the chicken pox virus remains dormant in the body long after the symptoms of chicken pox have vanished. Years later, this virus can reactivate and travel along the nerves to the skin, where itchiness occurs and pain ensues from a blistered rash that lasts up to two weeks. Shingles and chicken pox are caused by the herpes zoster virus, while another type of herpes virus, herpes simplex, causes the itchy, blistered rash associated with genital herpes and cold sores.
DermNet NZ lists numerous bacterial and viral diseases that can cause widespread rashes, called exanthems, which are accompanied by systemic signs such as fever. In some of these cases, including Kawasaki's disease and rubella, the rashes can cause itchiness.
Psychological Factors
People may report itchiness and rash when they experience various forms of psychological distress, a phenomenon sometimes referred to as "breaking out in hives." The American Academy of Allergy, Asthma and Immunology explains that the brain chemicals serotonin and norepinephrine can prompt cases of psychogenic itch. Additionally, people may suffer from a groundless belief that their body is infested with parasites, a condition called delusional parasitosis. Such patients may experience itching because of their psychological condition. Itching and rash caused by psychological factors may be treated with antidepressants and antipsychotic drugs.
