Abstract: First and second compounds are provided, where the first compound is administered to a mammalian host into blood for covalent bonding to blood components, where the components have an extended lifetime in the blood stream. The first compound comprises an active functionality and an agent of interest or a first binding entity. A second compound may be subsequently administered to the patient, which comprises a second binding entity, complementary to the first binding entity and an agent of interest. By virtue of binding to long-lived blood components, the half-life of the agent of interest is greatly extended in vivo.