Early T cells keep autoimmunity at bay A major challenge faced by the immune system is to react to foreign substances, such as microbes, while simultaneously tolerating the self. Upsetting this balance leads to autoimmunity. Regulatory T cells (T regs ), are a subset of immune cells that help to maintain this balance. Yang et al. found that murine T reg cells generated very early in life are distinct from those generated in older animals and play an especially important role in keeping autoimmunity in check (see the Perspective by Tanaka and Sakaguchi). These changes are due to differences in the way T regs develop in the thymus in newborn versus adult mice. Science , this issue p. 589 ; see also p. 506

Regulatory T cells generated early in life play a distinct role in maintaining self-tolerance