Scientists have long known that the body's immune response depends on proper direction of cells through the bloodstream and into the tissues that need them. But exactly how this cell trafficking occurred was unknown. Now new research, reported today in the Proceedings of the National Academy of Sciences, reveals that just as ZIP codes direct mail to particular communities, special molecules that sit on cell surfaces guide the cells through the bloodstream to their tissue destinations.

Before it can slip into the target tissue, a cell exiting the bloodstream must first adhere to the vessel wall. Blood cells contain more than 100 adhesion molecules, yet only a handful of these form bonds that can admit the cell into the surrounding tissue. According to the new study, these so-called Goldilocks molecules possess very specific chemical and mechanical properties that enable the formation of bonds of a particular strength and duration. "For cells to exit the bloodstream you need just the right adhesion," explains lead author Daniel A. Hammer of the University of Pennsylvania. "It can't be too tight, or the cells will bind to the vessel wall and never let go. It can't be too weak, or the blood cell will just pass on by."

The team employed computer simulations to explore the adhesion between a blood-borne cell and a surface akin to a vessel wall. Considering their success, Hammer says, other researchers interested in studying the chemical and mechanical properties of bonds may also want to utilize this computational method. He further notes that the technique could one day be used to design bonds with tailored mechanical properties.