Immunoglobulin G subtype 4, or IgG4, is a clinical immunological test that is related to the state of humoral immune function and can help doctors diagnose related diseases. The human immune response is a complex process. The invading substance that provokes the body’s immune response is called an antigen, while the body’s own defense against the antigen is an antibody, or immunoglobulin. Immunoglobulin G is an important component of human immunoglobulin tissue and plays an important role in many immune-related activities such as resistance to viruses, bacteria and immune regulation, etc. Immunoglobulin G can be divided into four subtypes according to its structure, including IgG1, IgG2, IgG3 and IgG4. It can help to determine the specific situation of the patient. If the test index is higher than normal, it is commonly found in diseases such as systemic lupus erythematosus, rheumatoid arthritis and bronchial asthma, but it can also be seen in some hematological diseases and liver diseases, but the specificity is not strong. In general, reduced IgG4 is less common and mostly occurs in malnutrition due to some diseases, such as liver cirrhosis.