The purpose of immunohistochemistry for tumors is to help determine the type of tumor as well as the origin and prognosis of the tumor, depending on the patient’s individual situation. Immunohistochemistry is a means of detecting proteins, based on the principle that antigens and antibodies react with each other to produce light and color, which enables the quantification and localization of proteins. A positive immunohistochemistry test may indicate the presence of a malignant disease, such as small cell lung cancer. Secondly, immunohistochemistry can help doctors determine the origin of the tumor cells because different tumor cells have different specific antibodies on their surface, such as epithelial, mesenchymal, endocrine, lymphatic, and neural sites. In addition, immunohistochemistry can play a role in formulating treatment plans. With the results of immunohistochemistry, doctors can choose appropriate treatments for patients, such as endocrine therapy, targeted therapy, and so on. Immunohistochemistry is an important pathology diagnostic tool, patients should try to choose a large professional hospital to carry out the examination, and strictly follow the doctor’s arrangements, maintain a relaxed state of mind.