The mechanism of occurrence is due to the combination of antigen and antibody to form immune complexes deposited in the blood vessel wall, which activates complement and leads to inflammation in and around the capillary and small vessel walls, increasing the permeability of the vessel wall, resulting in various clinical manifestations. Clinical manifestations The disease occurs in children and adolescents and may start with fever, headache, arthralgia and general discomfort. The skin lesions may appear as pinhead to soybean-sized petechiae, bruises or urticaria-like rash, and in severe cases, blisters, hemorrhages or even ulcers may occur. The skin lesions are usually found on the extremities, especially on the lower extremities and buttocks. The lesions are symmetrically distributed, appear in batches, and are prone to recurrence. If there is only skin damage, it is called simple purpura; if there is abdominal pain, diarrhea, blood in stool, or even gastrointestinal bleeding, it is called gastrointestinal purpura; if there is joint swelling, pain, or even joint effusion, it is called joint purpura; if there is hematuria, proteinuria, or kidney damage, it is called renal purpura. The diagnosis is not difficult when there is purpura of both lower limbs, with abdominal pain, arthralgia or kidney damage. However, when the systemic symptoms appear before the skin purpura, it is easy to misdiagnose as rheumatoid arthritis or acute abdomen, and it needs to be clinically distinguished from these diseases and other types of purpura and vasculitis. The main reason for this is that it is a very important factor in the development of the disease. 2, drug treatment (1) antibiotics with infection factors can be used appropriate antibiotics. (2) antihistamines are suitable for simple purpura, can use rutin, vitamin C, calcium, aniloxin or haemostasis at the same time. (3) Aminophenyl sulfone is effective in early selection. (4) Glucocorticoids are suitable for severe skin damage or joint type, abdominal type, renal type purpura. (5) Immunosuppressants in patients with recalcitrant chronic nephritis, cyclophosphamide or azathioprine can be used. It can be used in combination with glucocorticoids. (6) symptomatic treatment fever, arthralgia can use antipyretic analgesic such as indomethacin, fenbid; abdominal pain with scopolamine orally or intramuscularly, or atropine intramuscularly; (7) other treatments have been reported to cure allergic purpura with cimetidine, compound salvia injection, rehmannia, dextran-40, dipyridamole, etc. respectively. 3, plasma replacement This method can effectively remove the immune complexes in the blood circulation, thus preventing vascular obstruction and infarction. It is suitable for abdominal and renal type patients with large amount of immune complexes in plasma.