The root cause of allergic purpura is a defect in the body’s immune mechanism. The allergic source is only the trigger for the disorder. First of all, the allergen check is not comprehensive, there are more than 20,000 kinds of allergens, at present only a few hundred kinds can be found, some hospitals can only check a few dozen or a dozen, there are many things that can not be avoided. For example, some patients are allergic to dust mites in the air, flour, rice, etc., which cannot be avoided. Second: Some patients have checked the food they are not allergic to, but after eating it, their condition will still be aggravated, so the allergen test is not very accurate, and they cannot fully follow the allergen test results to formulate matters that need attention. The third: allergens are not set in stone. Allergic purpura onset period, in a hypersensitive state, this time allergic to many things, including things that are not usually allergic, not only allergic to a specific few items, found after not contact will not out purpura again. For example, when severe urticaria, at this time, allergy tests with saline may be allergic. When the purpura subsides, the body is in a hypoallergenic state, and the items that you were allergic to at the onset, you are not allergic again. Fourth: General hospital allergen check is more expensive. For the above reasons, allergens are generally not routinely checked.