immunoreactivity

Immunoreactivity is the property of immune components to recognize and react with specific antigens. It most often refers to antibody–antigen interactions or T‑cell receptor recognition, and is a key concept in both immunology and pathology. In laboratory practice, immunoreactivity describes the observable reactivity of a sample to an immune probe, such as an antibody or serum with target antigens, and is assessed in assays including immunohistochemistry, immunofluorescence, ELISA, Western blot, and flow cytometry.

In immunohistochemistry and related techniques, immunoreactivity denotes the presence and localization of a target antigen within tissue or cells, indicated by antibody binding and an detectable signal. Scoring typically combines measures of staining intensity and distribution. Positive immunoreactivity supports the presence of the antigen, while negative immunoreactivity suggests absence or below-threshold levels.

Several factors influence immunoreactivity. Epitope accessibility and preservation affect antibody binding; fixation and tissue preparation can mask or alter epitopes; antigen retrieval methods (such as heat-induced or enzymatic retrieval) aim to restore antigenicity. Antibody affinity, specificity, lot variation, detection systems, and background staining also impact results. Controls, including positive, negative, and isotype controls, help interpret immunoreactivity.

Distinctions are important: immunoreactivity describes binding/reactivity, whereas immunogenicity refers to an antigen’s capacity to provoke an immune response. Immunoreactivity is widely used in diagnostic pathology and research to assess protein expression, localization, and antibody performance, and to interpret diagnostic or experimental findings.