Species Reactivity

Species reactivity is the entry on a datasheet stating which species’ version of a target an antibody or assay kit detects — human, mouse, rat and others. It derives straight from the immunogen sequence: antibodies bind epitopes, and when that sequence stretch is conserved across species, the reagent will generally cross-react.

Tested or merely predicted?

This difference is exactly why the field deserves a careful look. "Tested" means the supplier actually checked the reagent on samples from that species in a named application. "Predicted" is an inference from sequence homology — frequently given as a percentage identity — and is a hypothesis rather than evidence. Homology can also mislead in both directions: one substitution within a short linear epitope may eliminate binding despite 95% overall identity, while a completely conserved epitope may yield reactivity in a species the supplier never checked.

There are two more pitfalls. Reactivity may depend on the application: an antibody can perform in western blot on rat samples yet fail in immunohistochemistry, since denatured and native epitopes are not the same. And the host species counts — a mouse monoclonal applied to mouse tissue generates strong background unless a mouse-on-mouse blocking system is used. The same conservation reasoning applies to peptides as such, where rodent and human native sequences often diverge.

Related terms and material

polyclonal vs monoclonal · ELISA · in vivo. Reference material: antibodies, assay kits.