= Cross-reactivity
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<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. <Immunobiology: The Immune System in Health and Disease, 5th edition by Charles A. Janeway, Jr. and others/Appendix I. Immunologists' Toolbox>[Immunobiology, 5th edition, Appendix I, antiserum generation: cross-reactive antigens and absorption]{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.
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