The CD15 IHC test detects the presence of CD15 antigen (Lewis X antigen) in tissue sections. CD15 is a carbohydrate antigen expressed on the surface of granulocytes, certain monocytes, and Reed–Sternberg cells in classical Hodgkin lymphoma. The test uses a specific monoclonal antibody to visualize CD15 expression via a chromogenic reaction, highlighting positive cells under light microscopy.