Bringing in the agent of your own destruction Cells need mechanisms to detect and disable pathogens that infect them. Tam et al. now show that complement C3, a protein that binds to pathogens in the blood, can enter target cells together with the pathogen. Once inside the cell, the presence of C3 triggers both immune signaling and degradation of the internalized pathogen. The discovery of this pathway reveals that cells possess an early warning system of invasion that works against a diverse array of pathogens and does not require recognition of any specific pathogen molecules. Science , this issue 10.1126/science.1256070