Once nanoparticles enter the bloodstream, they are quickly opsonized, meaning that proteins in the blood mark them for recognition by the RES. Macrophages, primarily located in the liver, spleen, and lymph nodes, then engulf and remove these nanoparticles from circulation. This process is a significant obstacle in the use of nanoparticles for drug delivery, as it reduces the amount of the therapeutic agent that reaches the target tissue.
