Neutrophil-mediated killing of C albicans. Two independent and distinct neutrophil-mediated killing mechanisms of C albicans were characterized. The killing of unopsonized C albicans depends on CR3 recognition, PI3K, and CARD9 signaling but is independent of NADPH oxidase activation. The killing of opsonized C albicans involved FcγRs, signaling through PKC and requires ROS formation by the NADPH oxidase system. Syk signaling is essential in both pathways for the killing of unopsonized and opsonized C albicans killing.