Allergies are the result of the body overreacting to foreign substances (allergens) that are supposed to be harmless. People who develop allergies mainly have an allergic constitution, which is genetically related. The immune system of such a person’s body readily produces immunoglobulin E (IgE), and IgE meets the allergen in an allergic reaction. The allergen enters the body through the nose, lungs, skin, and mouth, and the immune system mistakenly perceives it as dangerous and produces IgE that can fight the allergen to defend against it. The immune system has a memory, and when the body encounters the allergen again, an allergic reaction occurs with allergic symptoms – swelling, redness, and itchiness – manifesting in the skin, nose, eyes, respiratory tract, and digestive tract. Eyes: allergic conjunctivitis, red and itchy eyes, tearing, soreness, etc. Respiratory system: in the nasal cavity is called allergic rhinitis, manifested as sneezing, runny nose, nasal congestion, sometimes itchy nose and throat, teary eyes, dark bags under the eyes (allergic dark circles), in the sinuses often combined with chronic sinusitis; invasion of the lower respiratory tract, the lungs appear coughing, wheezing, chest tightness, breath-holding, called bronchial asthma. Skin: may appear atopic dermatitis or eczema, eczema symptoms when mild manifest as dry skin, a little peeling, if severe can manifest as redness, long blisters or moist with oozing body fluid. It can also manifest as hives, red itchy lumps of skin that last a short time and usually disappear within 24 hours. Gastrointestinal tract: Rapid reactions to allergenic foods begin to show symptoms within minutes of entering the mouth, or some allergic reactions can drag on for several hours, and the faster they occur, the more intense and severe the reaction. First of all, the lips and tongue become numb and itchy, the lips and mucous membrane inside the mouth experience symptoms and nausea, vomiting and diarrhea. Food allergy can also occur as skin symptoms such as hives, or respiratory symptoms such as wheezing and coughing. In the most severe cases, it can lead to severe allergic reactions, which can cause dizziness, shortness of breath, wheezing, breath-holding, palpitations, deficiency and sudden drop in blood pressure during the attack, and such severe allergic reactions throughout the body can be life-threatening. These are also the systemic manifestations of allergic diseases. Patients can have one manifestation such as allergic rhinitis or multi-system manifestations such as allergic rhinitis combined with asthma and allergic conjunctivitis. These manifestations vary from person to person, with large individual differences. They can also have different allergic manifestations at different ages, such as eczema in infancy, recurrent cough and wheezing in early childhood, and recurrent nasal congestion and runny nose in old age.