I am often asked what is the difference between clinical tests and pathology tests. How do pathologists make a diagnosis of cancer? And what kind of immunohistochemistry is used in the pathology department? Clinical testing is the examination of blood, urine, stool, secretions and excretions from patients with the help of advanced testing technology to examine the pathology, chemistry, pathogenesis and microscopic morphology of the specimens, in order to meet the needs of clinicians for screening diseases with simple and rapid results. And pathological examination is a study of disease etiology, pathogenesis, morphological and structural changes and the resulting functional changes of a bridge between basic medicine and clinical medicine. The task of pathological testing is to provide a clear pathological diagnosis based on surgically excised specimens, various biopsies collected from the patient’s lesion site, puncture and exfoliative cytology, to provide possible etiological evidence or clues; to provide relevant prognostic factors of the disease, etc. With the above preliminary understanding of diagnostic pathology, it is not difficult for us to know how a pathologist makes a diagnosis of cancer. When a tumor patient (cancer patient) goes to the hospital, a surgeon or other doctors will cut a small piece of tissue (flesh) from the tumor site or use a needle to remove some tissue and send it to the pathology department. The tissues sent to the pathology department should be fixed with chemical reagents, dehydrated and embedded in paraffin, and made into very thin glass slices, which can be observed under the microscope only after staining with some dyes. The pathologist will make a diagnosis of whether the tissue sent for examination has tumor cells and whether it is highly malignant or low malignant based on the morphological changes of the tissue cells under the microscope. But unfortunately, many tumor cells look very similar under microscope, so it is difficult for pathologists to determine the nature of tumor and make differential diagnosis completely and accurately, because only when the nature of tumor is clear, clinicians can develop the most effective treatment plan for different tumors, so how can we solve this problem? With the progress of the society and the development of science, some specific substances on the surface of tumor cells and in the cells have been studied at the molecular level, and the expression of these substances can be seen by pathologists under the microscope through a kind of inspection method, and they can make the differential diagnosis and preliminary characterization of tumors according to the different substances expressed by different tumors. This method is called immunohistochemistry. Immunohistochemistry is a method of accurately and specifically expressing substances at the molecular level that may be associated with cancer cells, bacteria, viruses, and so on. Since the successful production of these specific antibodies so that tissue structures can be revealed from the molecular level, these specific antibodies can bind to specific structures in tumor cells, these antibodies are called monoclonal antibodies. These monoclonal antibodies are incubated on glass slides of tissue sections and react specifically with specific target cells in the tissue. The antibodies bound in the tissue section are then stained with a dye that shows them in brown or red to facilitate the pathologist’s observation and judgment under the microscope to make a correct pathological diagnosis of the tumor tissue. The whole process of this technique is called the immunohistochemical method.