How does complement activation kill pathogenic bacteria?

The human immune system punches holes in bacteria using a protein complex called the membrane attack complex (MAC), but no one knows exactly how those holes manage to kill the cell. This matters because MAC is the immune system’s last-resort weapon against Gram-negative bacteria—pathogens like *E. coli* and *Klebsiella* that cause sepsis and hospital-acquired infections. The problem is a geometric puzzle: MAC pores are too shallow to span the bacterial envelope, which has two lipid membranes separated by a peptidoglycan layer. The researchers suspect the answer lies in how pores cluster and how the local envelope structure changes under attack. They will build artificial cell mimics to test this, combining cryo-electron microscopy with single-molecule imaging. If this research succeeds, it will reveal why some bacteria resist MAC killing—a gap in fundamental understanding that currently limits efforts to restore or enhance this natural immune mechanism. This is fundamental science: the immediate payoff is a clearer picture of how our own bodies work. But historically, understanding immune pores has led directly to therapies for complement-driven diseases, and knowing how bacteria evade them could eventually inform new antibiotics or immune-boosting treatments.