Immunohistochemistry. A, Schematic representation of the general immunohistochemistry method. (1), The cellular antigen is recognized by the specific primary antibody, (2), A secondary antibody, (3), directed against the primary antibody, reacts with the enzymatic complex to create the chromogen, (4), The final product allows the visualization of the cell containing the antigen. B and C, A metastatic carcinoid to the orbit seen by hematoxylin-eosin (H&E) staining (B) shows bland epithelial characteristics. In C, a chromogranin antibody highlights the neuroendocrine nature of the cells.