1, have allergic constitution or have a more certain allergen trigger 2, clinical manifestations: prodromal symptoms: 1 to 3 weeks before the occurrence of purpura have symptoms such as low fever, upper respiratory tract infection and general discomfort; typical skin purpura and corresponding skin lesions; the course of the disease may have abdominal pain, joint symptoms or kidney damage symptoms. Platelet count, platelet function and clotting time were normal, and capillary fragility test was positive. Histological examination: a more homogeneous allergic vasculitis may be seen in the skin or tissues of the affected area, with a large number of post-capillary small veins infiltrated by leukocytes, fibrous-like necrosis and erythrocytes leaking out of the vessels, and the vessel wall may have a source of necrosis and proliferation of epithelial cells. In addition to vasculitis caused by other diseases, cold globulin syndrome, benign hyperglobulin purpura, circumscribed capillary dilated purpura, pigmented purpura tinea-like dermatitis, etc.