Immunoreactivity is a word pathologists use to describe the results of a test called immunohistochemistry. This test is performed to see if the cells in a tissue sample make a protein of interest.
Cells that make the protein may be described as reactive or positive. Cells that do not produce the protein may be described as non-reactive or negative. If immunohistochemistry was performed, the results of the test will usually be described in the microscopic description or comments section of your pathology report.