This is the study of molecular mechanisms underlying the functions of the immune system, specifically on the nature of antibodies, antigens and their interactions. This study further entails the use of antibodies in the labelling of epitopes of interest in cells (Immunocytochemistry) or tissues (Immunohistochemistry). These techniques are used for the diagnosis and prognosis of diseases and tumours with the development of locus – specific monoclonal antibodies. Such application has improved the system of diagnosing diseases and made way for the exercise of preventive measures against the development of tumours.