No one expected that a peach would bring such a big trouble to Xiao Li. Not long after he gobbled up the peach, he had vomiting and diarrhea, even shortness of breath, and a lot of red spots grew on his arm joints. His family rushed him to the hospital, where the doctor examined him and concluded that it was an allergy. Fortunately, he was sent in time, otherwise he might have been in danger of dying. Xiao Li was baffled: He used to enjoy eating peaches, but why did he have a big problem this time? Under normal conditions, the human immune system can play a role in defending against external bacteria and viruses, but they can also have moody times and react abnormally to many substances, such as pollen, fur, drugs and so on. When the immune system is repeatedly exposed to them, it will be impressed, and when these substances come into contact with the body again, the immune system will be as “hyper” as when it attacks bacteria and viruses, resulting in allergies. Allergens can be classified as airborne (e.g. pollen, fur), food (e.g. milk, seafood), drug (e.g. penicillin), contact (caused by exposure to a substance) and occupational allergens. Allergy may induce many symptoms, such as airborne allergy may lead to allergic rhinitis, asthma r contact allergy may cause contact dermatitis r food allergy will cause gastrointestinal symptoms. The first time an allergy occurs, most people are unaware of the allergenic culprit behind the scenes. In acute attacks, allergy symptoms are often more severe (Xiaoli’s symptoms are typical) and must be treated in the emergency department of a nearby hospital as soon as possible. Acute allergy can easily lead to anaphylaxis or suffocation due to laryngeal edema and airway blockage, which is very dangerous and requires action as soon as possible. After the acute symptoms are eliminated, it is best for the patient to go to the hospital to have the allergen measured. There are usually two methods, one is an in vivo test, including skin prick test and patch test r the second is a blood test. This will prevent recidivism as much as possible. As for why some people who were not allergic to a substance before are now suddenly allergic again? There are usually three possibilities. First, the allergic reaction is gradual and sometimes it takes many exposures to the allergen before symptoms appear r Second, even if it is the same type of thing, the one that caused the allergy this time may be different from the one that was previously exposed r Third, the thing itself is fine, but the other substances attached may cause the allergy. The most important thing for people who know what substances they are allergic to is to try to avoid contact with the allergen. However, there are some allergens in nature that cannot be prevented, such as dust mites, pollen, molds, etc. People who are allergic to them can consider desensitization treatment.