Figure 1. Phagocytosis. (1) Plasma proteins are adsorbed on to the particle (opsonization). (2) The opsonized particle binds to surface receptors on the macrophage. (3) Pseudopodia extend and engulf the particle. Binding to many receptors brings about a ‘membrane-zippering’ effect. (4) The particle is completely enclosed within a phagosome. (5) The phagosome is acidified. Tubular protuberances are formed, into which the receptors segregate. (6) Vesicles containing receptors are recycled to the cell surface. The phagosome fuses with one or more lysosomes and particle digestion occurs.

Figure 2. A blood protein (opsonin) rejected by a sterically stabilized polymeric nanoparticle coated with polyethylene glycol (PEG) chains.

Figure 3. Double recognition of peptide and major histocompatibility complex (MHC) by T cells. (a) Peptide carried on MHC I recognized by a CD8-bearing cytolytic T cell; binding leads to destruction of the target cell. (b) Peptide carried on MHC II recognized by a CD4-bearing helper cell; binding leads to T-cell activation which enables it to stimulate the corresponding B cells for antibody production. APC, antigen-presenting cell; Tc, cytotoxic T cell; TCR, T-cell receptor.