Macrophages are big, fuzzy-looking immune cells that move through the blood and tissues, engulfing and destroying any foreign invaders, such as bacteria and dead or damaged cells, they encounter. In July, Holger Kress and colleagues from the European Molecular Biology Laboratory in Heidelberg, Germany, reported that macrophages don’t just wait to bump into their victims. At the cell surface, they form thin protrusions, called filopodia, that wave around like fishing lines. If a filopodium bumps into an invader, the “line” retracts quickly, helping the cell swallow up the object.
Initially, Kress wasn’t looking at the filopodia at all. He was using microscopic beads to watch what happened when a macrophage encountered a foreign object. But he was focusing on the smooth regions of the membranes, where the analysis of that encounter would be straightforward. Then one of his beads bumped into a filopodium and—wham!—the protrusion retracted, taking the bead with it. With that, he saw the process in a whole new light.
Kress, now a postdoctoral researcher at Yale, thinks the cell uses filopodia to increase the size of the area it can explore in a given time. “The cells are likely able to capture more bacteria per period of time than if the membrane were completely flat,” he says.
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