FIGURE 17-19 Phagocytosis and actin dynamics. Actin assembly and contraction drives the phagocytic internalization of particles. Shown here is the phagocytosis and degradation of a bacterium by a leukocyte. An invading bacterium is coated by specific antibodies to a cell-surface protein in a process known as opsonization step 1. The Fc region of the bound antibodies is displayed on the bacterial surface and recognized by a specific receptor, the Fc receptor, on the leukocyte surface step 2. This interaction signals the cell to assemble a contractile actin structure that results in the internalization and engulfment of the bacterium step 3. Once it has been internalized into a phagosome, the bacterium is killed and degraded by enzymes delivered from lysosomes step 4.