Histopathology refers to the microscopic examination of various forms of human tissue. Specifically, in clinical medicine, histopathology refers to the examination of a biopsy or surgical specimen by a pathologist, after the specimen has been processed and histological sections have been placed onto glass slides. This contrasts with the methods of cytopathology, which uses free cells or tissue fragments. Histopathological examination of tissues starts with surgery, biopsy, or autopsy. The tissue is removed from the body of an organism and then placed in a fixative that stabilizes the tissues to prevent decay. The most common fixative is formalin, although frozen section fixing is also common. To see the tissue under a microscope, the sections are stained with one or more pigments. The aim of staining is to reveal cellular components; counterstains are used to provide contrast. Histochemistry refers to the science of using chemical reactions between laboratory chemicals and components within tissue. The histological slides are then interpreted diagnostically and the resulting pathology report describes the histological findings and the opinion of the pathologist. In the case of cancer, this represents the tissue diagnosis required for most treatment protocols.