Contact dermatitis is an inflammatory reaction of the mucous membrane of the skin due to contact with external substances, such as chemical clothing, cosmetics, drugs, etc. It is characterized clinically by edematous erythema at the site of contact. It is characterized clinically by the occurrence of sharp-edged damage at the site of contact, ranging from edematous erythema in mild cases to papules, bumps and even large scars in more severe cases, and epidermal loosening and even necrosis in more severe cases. If the cause can be removed early and treated appropriately, it can heal quickly, otherwise it may turn into eczema-like dermatitis. There are many substances that can cause contact dermatitis. Some substances are allergenic at low concentrations and irritating and toxic at high concentrations. According to its nature can be divided into 3 categories: 1, animal: animal toxins, insect secretions, toxic hair, etc. 2, plant-based: pollen, plant leaves, stems, flowers and fruits, etc. 3, chemical: is the main cause of contact dermatitis. There are mainly metals and their products, plastics, rubber, spices, etc. Contact dermatitis is a typical contact delayed hypersensitivity reaction, belonging to the type of hypersensitivity reaction, usually caused by contact with small molecules of semi-antigenic substances, such as paint, fuel, pesticides, cosmetics and certain drugs (sulfonamide and penicillin), small molecules of semi-antigens combined with body proteins into complete antigens, uptake by Langerhans cells and presented to T cells, so that their activation and differentiation into effector T cells, the body again Contact dermatitis can occur when the body is exposed to the corresponding antigen, resulting in localized skin redness, rash, blistering, and in severe cases, exfoliative dermatitis. It is an antigen-induced T-cell immune response. The disease has an acute onset with well-defined edematous erythema, papules, and blisters of varying sizes at the site of contact; the walls of the blisters are tense, the fluid in the blisters is initially clarified, and pustules form after infection; the blisters rupture to form a vesicular surface and even tissue necrosis. If the contact material is gas, dust, lesions mostly occur in exposed parts of the body, such as the back of the hands, face, neck, etc., dermatitis boundaries are unclear. Sometimes the contact is brought to other parts of the body, such as the vulva, waist, etc., due to scratching, similar dermatitis can occur. If the body is in a highly sensitive state, the lesions are not only limited to the contact area, but can be very wide and even spread throughout the body. Self-perceived symptoms may range from itching to burning or swelling in severe cases. Systemic reactions include fever, chills, headache, nausea and vomiting. The course of the disease is limited. Removal of the cause can be cured after 1-2 weeks of appropriate treatment, but if re-exposed to the allergen can be re-emerged, repeated exposure, and repeated attacks. If not treated properly can develop into subacute or chronic inflammation with local tinea-like lesions: 1. History of exposure to irritants or allergens. 2. The rash often occurs at the site of contact with the irritant. 3, the rash form often varies depending on the nature of the contact, such as allergens are often clear edges, erythema, papules, blisters, can also occur in the self allergy; such as irritants are often red, swollen, blistering or blistering, erosion and even necrosis can occur. 4, there is itching and burning sensation, heavy with pain, fever and other systemic symptoms. 5, the course of the disease is self-limiting, certain allergens caused by the rash can be removed after the cause of l to 2 weeks can subside. 6. Positive skin patch test for allergens. Modern medical treatment of the disease is to stop contact with the allergen, immediately rinse the contact area with water, apply soothing anti-inflammatory and anti-itch topical and internal anti-allergy drugs. If necessary, add corticosteroids and choose antibacterial and internal medicine.