A major protein involved in clearing nanoparticles is called scavenger receptor A. This protein found on the macrophages in blood takes up nanoparticles, for example more than half of silica nanoparticles up to 500 nm. These nanoparticles bind to clusters of positively charged arginine and lysine residues that form when the receptor folds. Using such information scientists design new particle surfaces that better target or evade the receptor in order to shorten or prolong the circulation time of nanoparticles in the human system.

When nanoparticles circulate in blood, they are surrounded by a “corona” of proteins which is about 10 nm thick for many nanomaterials, and this thickness does not change with time. The corona composition depends on the protein concentration in the biological fluid and hence the corona composition varies in different parts of the body. The protein corona surrounding a particular nanoparticle depends on a number of parameters, including particle composition, morphology, size, charge, the duration of exposure and plasma concentration. Whether the protein corona just helps make particles biocompatible or the composition of the corona in some way matters for the cellular machinery that processes the nanoparticles is still not clearly known. Most nanoparticles show strong affinity to proteins. Smaller particles seem to bind proteins more strongly, but no clear trend is reported about the dissociation kinetics of nanoparticles and proteins according to size and type.