**Abstract**

Immunohistochemistry (IHC) is a process of selectively imaging antigens in cells or tissue sections by exploiting antibody specificity. This technique is widely used in diagnostic pathology and research experiments for tracking specific molecular markers characteristic of a particular cell type or cellular events such as cancerous cell development, cell proliferation, or apoptosis. Visualizing the target antigen following an antibody-antigen interaction is accomplished by different detection systems. In the simplest instance, primary antibody directly conjugated to an enzyme is responsible for both specifically binding to the antigen and catalyzing a color-producing reaction. Alternatively, complex detection systems could be designed to profoundly improve minimal detection level of the antigen. During the past years, there has been a considerable improvement in designing and introduction of new and highly sensitive detection systems. The choice of an IHC detection system is a compromise of a variety of variables including desired sensitivity, cost, and the time needed for an IHC staining to be performed. This chapter covers the immunohistochemistry detection systems with emphasis on their principle, history, advantages, and limitations and delineates factors needed to be considered for choosing an appropriate detection system for IHC applications.

**Keywords:** immunohistochemistry, antibody, detection systems, sensitivity, background
