WHITE BLOOD CELLS SORT THEMSELVES BY TYPE in an artificial environment with dimensions similar to those of human capillaries, the narrow blood vessels which connect arteries to veins. While red blood cells pass easily through a capillary (which is 4-5 microns in diameter), white blood cells (around 10 microns in diameter) must squeeze through the entrance with the help of complex hydrodynamic forces. Once inside the capillary, the cells stick somewhere on the capillary walls. Now, a research team (Bob Austin, Princeton, 609-258-4353) has constructed a series of artificial capillaries--microfabricated polyurethane channels, each with a width of 5 microns. Sending fluorescently labeled white blood cells through the channels, the researchers observed that T-lymphocytes (antibody-producing cells from the thymus) avoided sticking to regions occupied by groups of granulocytes (cells with grainy features surrounding the nucleus) and groups of monocytes (cells with a kidney-shaped nucleus). This unexpected self-sorting process suggests a sort of physical communication between the different types of cells. In addition, the artificial channels may potentially serve as a tool for identifying blood cell disorders. (Robert H. Carlson et al., 15 September Physical Review Letters; images at Physics News Graphics).