Hypersensitivity, also known as an allergic reaction, refers to an abnormal immune response, an overreaction by the body’s defense cells. Hypersensitivity reactions are studied to treat or prevent diseases caused by hypersensitivity reactions, such as hives and other diseases. Hypersensitivity reactions can be categorized into types I, II, III and IV. 1. Type I: characterized by rapid onset and rapid resolution. The more common ones are serum anaphylaxis, respiratory allergic reaction, drug anaphylaxis and so on. 2. Type II: common clinical diseases include neonatal hemolytic disease, blood transfusion reaction with ABO blood group incompatibility, etc. 3. Type III: characterized by local necrosis and congestive edema, common clinical diseases include serum sickness. 4. Type IV: characterized by slow reaction, which can be called delayed hypersensitivity reaction. Common clinical diseases include atopic dermatitis and allergic contact dermatitis. Patients with hypersensitivity reactions should be treated systematically under the guidance of a doctor and should not be treated blindly.