Cross-reactivity

Cross-reactivity occurs when an antibody also recognizes another antigen beyond the one against which it was raised. This may involve related features, but obvious overall similarity is not required. ^[ref]

For an imagined example, a paratope contacts a small surface patch shared by two otherwise different proteins. Both can bind even though their remaining surfaces differ. Increasing antibody concentration can reveal weak binding that was previously undetectable. Conversely, an extra Western blot band need not prove cross-reactivity: the intended protein may occur in multiple forms, or detection reagents may contribute background. The target-deleted and no-primary antibody comparisons under Western blot antibody specificity controls test different explanations.