PEGylation is the process of covalent attachment of polyethylene glycol
Polyethylene glycol
Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...

 polymer chains to another molecule, normally a drug or therapeutic protein. PEGylation is routinely achieved by incubation of a reactive derivative of PEG with the target macromolecule. The covalent attachment of PEG to a drug or therapeutic protein can "mask" the agent from the host's immune system (reduced immunogenicity
Immunogenicity is the ability of a particular substance, such as an antigen or epitope, to provoke an immune response in the body of a human or animal.- Immunogenicity :The ability to induce humoral and/or cell-mediated immune responses....

 and antigenicity
Antigenicity is the ability of a chemical structure to bind specifically with certain products of adaptive immunity: T cell receptors or Antibodies . Antigenicity was more commonly used in the past to refer to what is now known as immunogenicity, and the two are still often used interchangeably...

), and increase the hydrodynamic size (size in solution) of the agent which prolongs its circulatory time by reducing renal clearance.