CORRESPONDENCE chemokines. We propose that there are at least two ways to arrive at immune regulation. One possibility is that the action of cells in their effector phase (CD45RBloCD25+CTLA-4+), irrespective of TCR specificity, might curtail the expansion potential of any naïve cell with possible autoreactive function via competition for IL-2 and access to antigen-presenting cells. Alternatively, IL-7–driven homeostatic expansion of high-avidity naïve cells in a lymphopenic environment, followed by at least partial activation, could out-compete the expansion of other naïve T cells. As long as the competing naïve population does not consist of autoreactive cells, they might provide protection against pathological immune responses or autoimmunity.