PEG (polyethylene glycol) can increase the circulation time of drugs or drug carriers, such as lipid nanoparticles, by reducing their recognition and elimination by the immune system. Specifically, when PEG molecules are attached to the surface of the drug or drug carrier, they form a protective layer that shields it from detection by the body's immune cells. This is because PEG is a hydrophilic polymer that is not recognized as foreign by the immune system, and therefore it is not subject to the same clearance mechanisms that are responsible for eliminating foreign substances from the body. This protective effect is sometimes referred to as the "stealth effect" or "PEGylation", and it can significantly improve the pharmacokinetic properties of drugs or drug carriers, such as lipid nanoparticles, by increasing their circulation time in the bloodstream.