Our immune system has evolved to sense and eliminate harmful microbes, yet we tolerate beneficial bacteria that colonize our mucosal tissues. Chrysothemis Brown wants to understand the logic underlying the regulation of tolerance and immunity, and how balance is determined in different tissues and developmental stages. Brown and her team have identified a new immune cell that promotes tolerance to commensal microbes and food proteins. They now aim to define the principles and mechanisms underlying diverse immune responses to self and foreign proteins, with the goal of developing new therapeutics for autoimmunity, inflammatory diseases, and cancer.