There are very many allergic substances that lead to the occurrence of allergic purpura, so it is difficult to really identify as the direct causative factor. Through allergen testing, we can roughly understand the sensitive factors and try to control or avoid them for the purpose of preventing recurrence, which is generally considered to be related to the following factors: 1. Infection: such as infection with bacteria, virus or parasites. Infectious factors are the most common cause of the disease, and are also important factors that aggravate and cause recurrence of the disease. Streptococcus haemolyticus is a common bacteria, other bacteria such as Mycobacterium tuberculosis, Staphylococcus aureus, pneumococcus and S. typhi can also cause this disease. Viral infections are commonly influenza, measles, rubella, chickenpox, mumps and other viruses. Parasitic infections are most common with roundworms, followed by hookworms, filarial worms, schistosomes, flagellates, etc. 2, food factors: mainly fish, shrimp, milk, eggs, clams, chicken and mutton, etc.. It is caused by atopic body allergy to animal protein. In addition, chocolate and fava beans can also cause this disease. 3, drug factors: many drugs can cause the disease, common drugs are penicillin, streptomycin, chloramphenicol, erythromycin, sulfonamides, antipyretic and analgesic drugs such as aspirin, as well as gold, mercury, arsenic, bismuth preparations. 4, other triggering factors: such as cold stimulation, pollen inhalation, trauma, insect bites, vaccination, etc.. The majority of cases occur in spring and autumn, especially from September to December each year.