1. Protein to which a specific ligand or hapten is conjugated 2. Unlabeled protein introduced into an assay at relatively high concentrations which distributes in a fractionation process in the same manner as labeled protein analyte, present in very low concentrations. 3. Protein dodan kako vi se spriječilo nespecifično međudjelovanje reagenata s površinom, dijelovima uzorka i reagenata međusobno. 4. Proteini koje nalazimo u staničnim membranama, koji omogućuju prijenos liganada preko membrane.