Immunohistochemistry is a technique for identifying cellular or tissue constituents (antigens) by means of antigen-antibody interactions, the site of antibody binding being identified either by direct labelling of the antibody, or by use of a secondary labelling method. Immunohistochemical staining is widely used in the diagnosis of abnormal cells such as those found in tumors. Specific molecular markers are characteristic of particular cellular events such as proliferation or cell death (apoptosis).