Species reactivity is the datasheet field listing which species' form of a target an antibody or assay kit recognises — human, mouse, rat, and so on. It follows directly from the immunogen sequence: an antibody recognises an epitope, and if that stretch of sequence is conserved between species, the reagent will usually cross-react.
Tested versus predicted
The distinction is the whole point of reading the field carefully. "Tested" reactivity means the vendor ran the reagent against that species' sample in a stated application. "Predicted" reactivity is a sequence-homology inference — often quoted with a percentage identity — and it is a hypothesis, not data. Homology also fails asymmetrically: a single substitution inside a short linear epitope can abolish binding even at 95% overall identity, while a fully conserved epitope can give reactivity in a species the vendor never tested.
Two further traps. Reactivity can differ by application: an antibody may work for western blot in rat but not for immunohistochemistry, because denatured and native epitopes differ. And host species matters — a mouse monoclonal used on mouse tissue produces heavy background unless a mouse-on-mouse blocking system is used. The same conservation logic applies to peptides themselves, where rodent and human native sequences frequently differ.
Related terms and material
polyclonal vs monoclonal · ELISA · in vivo. Reference material: antibodies, assay kits.