Caltech researchers separate blood stem cells from other bone marrow cells and load them onto a newly developed microfluidic chip. Fluorescent signals indicate the presence of secreted proteins with one... In the battle against infection, immune cells are the body's offense and defense—some cells go on the attack while others block invading pathogens. It has long been known that a population of blood stem cells that resides in the bone marrow generates all of these immune cells. But most scientists have believed that blood stem cells participate in battles against infection in a delayed way, replenishing immune cells on the front line only after they become depleted.