Under normal circumstances, foreign substances enter the human body, and most of them face two fates. If recognized as useful or harmless substances by the human body, these substances will live in harmony with the human body and eventually be absorbed, utilized or naturally discharged. If these substances are identified as harmful substances, the body's immune system will react immediately to drive them away or destroy them, which is the protective effect of immune response. Immune response is one of the important functions of human defense system, but if this reaction is beyond the normal range, that is, when the immune system attacks harmless substances, this situation is called allergic reaction. Allergy is a disease, because unprovoked attacks can also damage normal body tissues, and even the immune system sometimes attacks and destroys the body's own tissues, which is very unfavorable to human health.