As we all know, allergic diseases are a common and frequent disease that seriously threatens human health, and its incidence is high in all countries in the world. So, how exactly does this disease occur? 1. Allergic constitution is an intrinsic factor of this disease. Allergic diseases occur as a result of exogenous allergens acting on the body. However, in the same environment and the same allergen action, why some people are prone to asthma, allergic rhinitis, eczema or hives, etc., while most people are unharmed? Modern medical research proves that it is determined by genetic factors. Allergic constitution, which determines the quality of an individual to have an allergic reaction (susceptibility). The source of allergic constitution is acquired by heredity. A family survey was conducted and found that 75% of the children of parents with allergies were allergic; if one of the parents was allergic, about 50% of the children were allergic; as for those with no allergic parents, only 3.8% of their children suffered from allergic diseases. This means that genetic factors play an important role in allergic diseases. Of course, the role of acquired factors cannot be ignored. 2, allergen exposure is the external condition of the disease. If a person is allergic to a certain drug, but if he never uses the drug, he will not have allergies to the drug in his life; another example is someone who has allergies caused by drinking milk when he was a child, and when he grows up, he will no longer drink milk and his allergies will be cured. However, when he starts to consume milk or dairy products again, his allergy may come back again. Therefore, allergy and exposure to external allergens are the two determining factors in allergic diseases. If you have allergies but are not exposed to allergens, you will not develop the disease; conversely, if you are exposed to allergens but are not allergic, you will not develop the disease. Since genetic factors play an important role in the development of allergic diseases, do all allergic diseases manifest themselves at birth? This is not the case. In one patient, although her daughter was allergic to milk in infancy, the patient herself did not develop symptoms of allergic rhinitis until she was 28 years old, while her father contracted bronchial asthma when he was over 50 years old. Some patients, especially those with asthma, have had asthma symptoms in childhood, which may resolve on their own as they get older, but at some point reappear, resulting in a recurrence of asthma. All these show that allergy is the internal cause of allergy occurrence, while allergen exposure is the external condition for the development. External causes work through internal causes, and the occurrence of allergic diseases is a concrete manifestation of this dialectical law.