Henoch-Schönlein purpura (HSP), also known as Immunoglobulin A vasculitis (IgAV), is a condition caused by inflammation of small blood vessels, particularly those in the skin, joints, intestines, and kidneys. The fundamental cause of HSP is an abnormal immune response that leads to the deposition of a specific antibody called Immunoglobulin A (IgA) in the walls of these small blood vessels. This immune complex deposition triggers inflammation, which damages the vessels and causes the characteristic symptoms of the disease.
The exact trigger for this immune response is not always clear, but it often follows an infection, especially an upper respiratory tract infection like a cold or sore throat. This suggests that the immune system, while fighting off an infection, mistakenly targets the blood vessels, causing inflammation. The immune complexes containing IgA accumulate in the vessel walls, leading to a type of inflammation called leukocytoclastic vasculitis, which primarily affects small vessels such as capillaries, venules, and arterioles.
Several factors contribute to the development of HSP:
– **Infections:** Most commonly, HSP occurs after infections of the respiratory tract caused by bacteria or viruses. These infections stimulate the immune system, which in some individuals reacts abnormally, producing excess IgA antibodies that form complexes and deposit in blood vessels.
– **Immune system dysregulation:** In HSP, the immune system’s regulation is disrupted, leading to the overproduction or abnormal handling of IgA. This causes immune complexes to form and deposit in vessel walls, provoking inflammation.
– **Genetic predisposition:** Some people may have a genetic tendency that makes their immune system more likely to react this way. Although HSP can affect anyone, it is most common in children between 4 and 7 years old, suggesting a developmental or genetic susceptibility.
– **Environmental triggers:** Besides infections, other environmental factors such as certain medications, vaccines, or even insect bites have been reported to precede the onset of HSP in some cases, possibly by stimulating the immune system.
– **Seasonal patterns:** HSP often shows seasonal peaks, particularly in the fall and winter months, coinciding with the higher incidence of respiratory infections, reinforcing the link between infections and disease onset.
The inflammation caused by IgA deposits leads to the typical symptoms of HSP:
– **Skin purpura:** The hallmark of HSP is purpura, which are red or purple spots on the skin caused by bleeding under the skin due to damaged blood vessels. These spots usually appear on the legs and buttocks.
– **Joint pain and swelling:** The inflammation can affect joints, especially knees and ankles, causing pain and swelling that may come and go.
– **Gastrointestinal symptoms:** Inflammation of blood vessels in the intestines can cause abdominal pain, vomiting, and sometimes blood in the stool.
– **Kidney involvement:** IgA deposits can also affect the kidneys, leading to hematuria (blood in urine) or proteinuria (protein in urine), which can be serious and affect long-term kidney function.
In children, HSP is often self-limiting and resolves on its own, but in some cases, especially when the kidneys are involved, it can lead to complications requiring medical treatment. Adults with HSP tend to have more severe disease and a higher risk of kidney problems.
In summary, Henoch-Schönlein purpura is caused by an abnormal immune reaction involving IgA antibodies that deposit in small blood vessels, usually triggered by infections or other immune stimuli. This leads to inflammation and damage to the vessels, resulting in the characteristic symptoms of purpura, joint pain, abdominal issues, and kidney involvement. The interplay of infections, immune system dysregulation, genetic factors, and environmental triggers all contribute to the development of this complex condition.
