Selection for Survival?

Abstract
Christophe Benoist and Diane Mathis The immune system responds to foreign substances more rapidly when it has encountered them before. In a Perspective in this issue, Benoist and Mathis discuss the "memory" cells that are responsible for this phenomenon and how new results reported in this issue by Tanchot et al. begin to define the stimuli required by these memory cells for proliferation and long-term survival.