Asymmetric control over immune activation by Tregs. Tregs have 2 key effects on controlling Th1 and Th2 responses. First, Tregs inhibit the proliferation of both subsets, reducing the magnitude of the immune response. Second, Tregs specifically enhance the apoptosis of Th2 cells by CTLA4 activity and reductions in IL-2 bioavailability. The net effect of these 2 forms of regulation is to shift the Th2/Th1 balance from being at parity in the absence of Tregs to heavily Th1 skewed in the presence of Tregs.