Rashes generally appear as part of an immune system response. The irritant, aggressor or allergen that triggers the response can be a bacteria, virus, fungus, chemical or, in some cases, simply an irritating substance, such as wool. Occasionally, rashes are indicators of an immune system malfunction, such as in the case of psoriasis, or a symptom of another underlying disease, as in the case of Lyme disease.
Psoriasis
Psoriasis is a chronic inflammatory skin condition that has an underlying genetic basis. It presents as a red rash with silvery scaly patches of skin that flake off. The name comes from the Latin word, "psora," which means "itch."
A psoriatic episode can be set off by certain triggers. Known triggers include injury to the skin, stress, infection, weather and lack of sunlight. Psoriasis often begins as a red spot on the skin that grows larger. As it grows, scales appear on the top of the rash. These break apart and shed in large quantities. If the scales are scraped off before they naturally shed, skin will bleed.
According to the American Academy of Dermatology, psoriasis occurs because of a faulty immune response. Normally, when the immune system detects an invading substance in the skin, it initiates an immune response that includes inflammation. In a person with psoriasis, the immune response occurs without an invading substance being present. Because of the inflammation, the body begins replacing skin cells at too rapid a rate. Whereas normal cellular turnover takes about 30 days, after a triggered immune response in a psoriatic episode, new cell production goes into overdrive, producing new cells every three to four days, according to the American Academy of Dermatology. The buildup of skin cells appear as the scaly psoriasis rash.
Eczema
Eczema is another chronic skin condition. It is characterized by an extremely itchy rash. When it presents during infancy, it is called "infantile eczema" and the rash usually appears on the scalp, forehead and cheeks. In some cases, the condition will completely resolve within the first 2 years of life, according to the American Academy of Dermatology. If it does not, the rash begins to appear on the backs of the knees and inside the elbow. When it develops during the teenage years, other areas are often affected, including the knees, elbows, hands, feet, face, neck and chest.
Lyme Disease
Lyme disease is a bacterial infection caused by a bite from a wood tick infected with the lyme borreliosis bacteria. The tick, which normally feeds off animals, such as deer, will attach to a passing human. It prefers warm, moist areas, so it may work its way to the armpits or groin area, where it inserts a probe into skin so as to feed off of blood, according to the Cleveland Clinic. Between one and four weeks after infection, a red rash appears. Usually, the rash will appear at the bite site, though later rashes can appear anywhere on the body. The Lyme rash often appears as a circular red rash surrounding paler skin, which accounts for its description as a "bull's eye rash." Left untreated, Lyme disease can cause serious, chronic health problems.
