Hypersensitivity reactions are divided into four types. Hypersensitivity reactions are abnormally high immune responses, as follows: 1. Type I hypersensitivity reactions, also known as anaphylactic reactions or rapid-onset hypersensitivity reactions. Common type I hypersensitivity reactions include anaphylaxis, drug-induced drug rash, food-induced allergic gastroenteritis, allergic rhinitis and bronchial asthma caused by pollen or dust mites, etc. 2. Type II hypersensitivity reactions, also known as cytolytic hypersensitivity reactions or cytotoxic hypersensitivity reactions, such as blood type incompatible transfusion reactions, neonatal hemolytic reactions and drug-induced hemolytic anemia are all type II hypersensitivity reactions. 3. Type III Hypersensitivity reactions, also known as immune complex hypersensitivity reactions or vasculitis hypersensitivity reactions, belong to type III diseases such as partial glomerulonephritis after streptococcal infection, chronic asthma, etc. Type IV hypersensitivity reactions, known as delayed hypersensitivity reactions, are a kind of immune cell-mediated pathological manifestations, and the common type is that chemicals bind to skin proteins or change their composition to become antigens that can sensitize T cells. Upon re-exposure to the antigen, the T cells become killer cells or release lymphokines, causing contact dermatitis. There is also a type called infectious metaplasia, which is caused by pathogens as antigenic stimuli and is seen in tuberculosis, syphilis, etc.