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 non-specific 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.