The word Immunohistochemistry derived from the words "immuno," refering to antibodies employed in the procedure, and "histo," which means tissue. It can be defined as the process of antigen or protein detection in the biological sample using antigen specific antibodies. Immunohistochemical staining is widely used in the diagnosis of abnormal cells such as those found in cancerous tumors. The antigen antibody interaction can be visualised by number of ways, the foremost common one being the use of a secondary antibody that conjugates with an enzyme to produce a coloured reaction, such as peroxidise. There are two types of immunohistochemical staining: direct and indirect method. Immunohistochemistry can be used as a diagnostic tool to assess which tumors are likely to respond to therapy, by detecting the presence or elevated levels of the molecular target.