Whether pathogens appear in your body at midnight or at noon could make a difference in your ability to fight them, a new study suggests. Ruslan Medzhitov, PhD, Sterling Professor of Immunobiology, and colleagues probed how macrophages—a type of white blood cell—responded to components of bacteria.The amount of an immune signaling molecule called IL-12p40 that the macrophages produced in response varied among cells. Some of this variation, the team showed, could be attributed to the phase of the circadian clock to which the cells were tuned. Just as the human body as a whole follows a circadian rhythm when it comes to such behaviors as sleep and eating, individual cells have fluctuations over a 24-hour time course. However, not all cells may be perfectly in sync all the time.The new findings, published on March 5 in Science Signaling, give scientists a new avenue to pursue in controlling immune responses. By altering levels of circadian genes, they may be able to alter immune function.