 Macrophages are key players in both the defense against parasitic infections and the suppression of the immune system. Recent studies have revealed that these cells can switch between two distinct modes of activation, either TH1-mediated or TH2-mediated, depending on the type of infection. While TH1-mediated activation leads to the production of pro-inflammatory cytokines and the destruction of parasites, TH2-mediated activation results in the production of anti-inflammatory cytokines and the suppression of the immune system. These findings suggest that macrophages play a critical role in the balance between protective and regulatory responses during infection. This article was offered by Stephen J. Jenkins and Judith E. Allen.