The movement. It is essential to the function of animal cells. A type of immune cell, these lymphocytes migrate toward inflammatory signals, as part of immune surveillance and defense. You see, the plasma membrane and the cell shape are constantly changing in order to achieve cell motility. How does this happen? Like a scaffold, this network of cytoskeletal fibers connect every corner of the cell and gives a cell its shape. Changes in cell shape rely heavily on one type of cytoskeletal protein, known as actin. The actin cytoskeleton is highly dynamic, and its filaments continuously assemble and disassemble throughout the moving cell. The building block of the actin cytoskeleton is a small globular protein called G-actin, which is normally bound to ATP as a monomer. These ATP bound G-actin monomers assemble or polymerize into filaments, known as F-actin, over three phases, called nucleation, elongation, and steady state. Nucleation occurs as three ATP bound G-actin monomers form trimers. An rapid linear polymerization occurs in a process called elongation. G-actin can bind reversibly to either end of the growing filament, known as the barbed or plus end, and the pointed or minus end, but monomers preferentially bind to the barbed end. Bound G-actin monomers undergo ATP hydrolysis, producing an increase of ADP bound G-actins at the pointed end of the filament. These G-actins disassociate more readily than the ATP bound G-actins at the barbed end of the filament. Thus, actin filaments undergo tread milling. As the reaction reaches a steady state, G-actin monomers are constantly added to the barbed end and removed from the pointed end of the filament. With the leading edge of a moving cell, actin filaments are continuously tread milling, branching off, and cross-linking. These activities push the plasma membrane, forming cell protrusions, such as wide blunt projections called lamellipodia, and narrow pointed projections called phyliopodia. Cylinder are formed by rapid elongation of parallel actin bundles, regulated by elongation factors such as foreman. Small cross-linking proteins, such as alpha-actinine, bind to these filaments and strengthen these parallel actin bundles. In lamellipodia, an actin binding protein complex, ARP-23, binds near the leading end of newly formed actin filaments and nucleates branching. Large cross-linking proteins, such as filament, bind to these actin filaments and support the structure of this branched network. This continuous branching off and cross-linking, strengthens the protrusive force needed to overcome the compressive force of the plasma membrane. As the cell moves forward, adhesions occur along the leading edge of the cell as actin bundles anchor to the surface via adhesion molecules. These newly formed adhesions are connected to old adhesions via contractile stress fibers, consisting of myosin filaments and antiparallal actin bundles. The cell is pulled forward, mainly by contractile forces generated by myosin acting on the actin filaments. As the contractile fibers contract, the rear of the cell comes forward. These dynamic changes in the actin cytoskeleton occur in a fraction of a second and work in concert to facilitate the continuous cycles of cell protrusion, adhesion, and contraction. This enables cell motility, ultimately allowing lymphocytes to perform their duty of immune surveillance and defense.