The pipette tip at the right is leaking a small amount of the peptide formyl-Met-Leu-Phe. Only bacterial proteins have formylated methionine residues, so the human neutrophil recognizes this peptide as the product of a foreign invader (discussed in Chapter 25). The neutrophil quickly extends a new lamellipodium toward the source of the chemoattractant peptide (top). It then extends this lamellipodium and polarizes its cytoskeleton so that contractile myosin II is located primarily at the rear, opposite the position of the lamellipodium (middle). Finally, the cell crawls toward the source of this peptide (bottom). If a real bacterium were the source of the peptide, rather than an investigator's pipette, the neutrophil would engulf the bacterium and destroy it. (From O.D. Weiner et al., Nature Cell Biol. 1:75–81, 1999. © Macmillan Magazines Ltd.)