When you look at the symptoms of allergies, no matter who you are, you will feel incomprehensible: ①Why are the symptoms different even though they are allergies? ② Why are some of the symptoms the same and the causes of the allergy different? A eats eggs to cause hives, B eats eggs to cause an asthma attack, and C’s asthma is caused by dust mites (found in dust more often) in the home. These are all called allergies (medically known as allergic reactions), but the symptoms are different and the substance causing the allergy varies from person to person. Allergies are a battle between the invaders and the defenders (the body’s immune system) who are trying to fight them off. But this battle is usually against pathogens such as bacteria and viruses that invade the body and worsen its condition. In contrast, when allergies occur, it is the defense army that is overly defensive against substances that are normally present in the environment, such as pollen and dust mites. Therefore, substances that do not cause any reactions in normal people, such as food and pollen, etc., are used as objects of removal to wage a fierce battle of attack and defense in the body, and manifest themselves as symptoms of allergy. Even though they are allergies, some people develop asthma while others develop hives. The key to solving the mystery of “allergy symptoms vary from person to person” lies in the path of allergen invasion: from where it enters and where it fights with the “defense army” in the body. The pathways of allergen invasion can be roughly divided into the following four ways: Pathway 1: Respiratory tract: dust mites, pollen, mold, animal dander, etc. in the air are invaded by the respiratory tract and adsorbed in the nasal mucosa, bronchial mucosa and lungs, where allergy occurs. Route 2: Skin and mucous membrane: contact with allergens invade in the contact area, in addition to the skin and the mucous membrane of the eyes, mouth, etc. Route 3: Gastrointestinal tract: Allergy is mainly caused by food, which enters from the mouth and contacts the mucous membrane of the gastrointestinal tract through the esophagus. Route 4: Injection: This is a case where the injected drug enters the body directly, and the allergen enters the skin, subcutaneous, muscles, blood vessels and other parts with the injection. From the above pathways, the appearance of allergy symptoms can be roughly surmised. If the battle occurs in the nasal cavity and bronchial tubes, it is called allergic rhinitis and/or asthma; and if the battle occurs in the digestive system, it causes diarrhea, vomiting, stomach pain, abdominal pain, etc.; if it is in the skin, it tends to cause eczema, hives, etc. However, these pathways are not absolute. For example, eggs can be a trigger for asthma attacks when they enter the body through the digestive system. In addition, the site where the body’s defense forces meet the invader is also a key issue. The defense forces have the ability to cause the body to produce immunoglobulin E (IgE), which causes allergies to occur. Depending on the individual, IgE tends to attach to specific sites such as the bronchial tubes, stomach and intestines, triggering allergies. The reasons for this are still not well understood. However, there are two elements that must be present to cause allergic symptoms: an allergen enters the body and the person who develops the allergy is allergic. On the basis of these two elements, various allergic symptoms can occur.