This animation was commissioned for the launch of the Centre for Advanced Molecular Imaging to highlight key themes of the research involved. In this scene, we follow a cytotoxic T cell (or killer T cell) as it patrols the body for potentially harmful pathogens. Using specialised T cell receptors (TCR) the immune cell probes antigens presented by the MHC-class I molecule on nearby cells and becomes activated when a foreign antigen is detected. A cascade of events follows, including production of inflammatory mediators and localised release of pore-forming proteins (perforin), compromising the target cell membrane and allowing the entry of destructive enzymes (granzymes), normally impermeant to the cell. Upon entry into the infected cell, granzymes induce cell death, thereby facilitating the clearance of the infection.