(a) AICD of T cells after recognition of tumor cells. Tumor recognition leads to activation of T cells and up-regulation of Fas and FasL on T cell surface, which results in the T cell killing of itself (“suicide”) and of other T cells (“fratricide”). (b) Proposed tumor “counterattack” model. Tumor cells express functional FasL and kill infiltrating Fas-expressing T cells via Fas-FasL binding, which leads to tumor escape. However, ligation of Fas expressed on innate immune cells such as neutrophils, macrophages and immature DCs by FasL expressed on tumor cells may also lead to release of multiple proinflammatory cytokines and chemokines, setting the stage for tumor rejection.