Ten years ago, Jonathan Kipnis decided to run an experiment to see how well mice can learn new things. He suspected that the immune system was important to cognition, and so he wanted to compare mice with normal immune systems to mice with deficient ones. Kipnis engineered mice that lacked T cells, a type of white blood cell that fights pathogens. At the time, Kipnis was getting his Ph.D. at the Weizman Institute in Israel, and the lab he was working in didn’t have the equipment necessary for the test. So he shipped off his mice–a group of normal ones and a group lacking T cells–to Ben-Gurion University. There, his colleague Hagit Cohen put the mice through their paces.

Cohen gave