ABSTRACT
Particles are taken up by cells through the process of phagocytosis or endocytosis. Phagocytosis is a major pathway for natural immune-related cells such as macrophages, monocytes, and neutrophils as a host defense mechanism against microorganisms, whereas endocytosis occurs in all types of cells. Phagocytosis of opsonized particles is initiated after cell membrane receptors recognize ligands such as Fcγ and C3bi on the particles. Actin molecules are assembled and the cell membrane extends around the opsonized particle like a zipper closure. On the other hand, endocytosis is initiated by invagination of the cell membranes. In either case, the cell membrane is nally pinched off and the particle is carried inside the cell. In general, particles >1 μm are taken up by phagocytosis and particles <200 nm by endocytosis. The differences between phagocytosis and endocytosis are schematically presented in Figure 8.1. Thus, nanoparticles are more likely taken up by cells via the endocytic pathway unless they are agglomerated [1].
