The teachings provide methods of delivering active agents that are conjugated with delivery systems that increase the half-life, and reduce the immunogenicity, of the active agents. Delivery systems and methods of making and using the delivery systems are also provided. The delivery systems have (i) a ligand that is selective for an endogeneous plasma protein in the serum of a subject; and, (ii) a linker configured for operatively attaching the ligand covalently to an active agent to increase the half-life of the active agent in the serum.