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 added to prevent nonspecific interaction of reagents with surfaces, sample components, and each other.

4. Protein found in cell membranes which facilitates transport of a ligand across the membrane.