The four main characteristics of Henoch-Schonlein purpura include:

• Rash (purpura). Reddish-purple spots, which look like bruises, are the most distinctive and universal sign of Henoch-Schonlein purpura. The rash develops mainly on the buttocks, legs and feet, but it can also appear on the arms, face and trunk and may be worse in areas of pressure, such as the sock line and waistline.
• Swollen, sore joints (arthritis). People with Henoch-Schonlein purpura often have pain and swelling around the joints — mainly in the knees and ankles. Joint pain sometimes precedes the classical rash by one or two weeks. These symptoms subside when the disease clears and leave no lasting damage.
• Gastrointestinal symptoms. Many children with Henoch-Schonlein purpura develop gastrointestinal symptoms, such as abdominal pain, nausea, vomiting or bloody stools. These symptoms sometimes occur before the rash appears.
• Kidney involvement. Henoch-Schonlein purpura can also affect the kidneys. In most cases, this shows up as protein or blood in the urine, which you may not even know is there unless you have a urine test done. Usually this goes away once the illness passes, but in a few cases, kidney disease may develop and even persist.

If your child develops the rash associated with this condition, take him or her to your doctor as soon as possible.

In Henoch-Schonlein purpura, some of the body's small blood vessels become inflamed, which can cause bleeding in the skin, abdomen and kidneys. Why this initial inflammation develops isn't clear. It may be the result of the immune system responding inappropriately to certain triggers.

Nearly half the people who have Henoch-Schonlein purpura developed the disease after an upper respiratory infection, such as a cold. Infectious triggers may include chickenpox, strep throat, measles and hepatitis. Other triggers may include certain medications, food, insect bites or exposure to cold weather.

Factors that may increase the risk of developing Henoch-Schonlein purpura include:

• Age. The disease affects primarily children and young adults, with the majority of cases occurring in children between 2 and 6 years of age.
• Sex. Henoch-Schonlein purpura is slightly more common in boys than girls.
• Race. White and Asian children are more likely to develop Henoch-Schonlein purpura than are black children.
• Time of year. Henoch-Schonlein purpura strikes mainly in autumn, winter and spring but rarely in summer.

For most people, symptoms improve within a month, leaving no lasting problems. But recurrences are fairly common.

Complications associated with Henoch-Schonlein purpura include:

• Kidney damage. The most serious complication of Henoch-Schonlein purpura is kidney damage. This risk is greater in adults with the condition than in children. Occasionally the damage is severe enough that dialysis or a kidney transplant may be needed.
• Bowel obstruction. In rare cases, Henoch-Schonlein purpura can cause intussusception — a condition in which a section of the bowel folds into itself like a telescope, which prevents matter from moving through the bowel. This is rare in adults.