Why did you get the flu this winter, but your co-workers didn’t? The answer, according to a new study of twins, may have less to do with your genes and more to do with your environment—including your past exposure to pathogens and vaccines. Our immune system is incredibly complex, with diverse armies of white blood cells and signal-sending proteins coursing through our veins, ready to mount an attack on would-be invaders. Everyone’s immune system is slightly different—a unique mixture of hundreds of these cells and proteins. But the main driver of this variation is unclear. Although scientists know that our immune system can adapt to our environment—that’s why vaccines work, for instance—it is also built by our genes.