Somewhat ironically, the instability of these plaques is believed to be largely caused by immune cells that try to clear them out. Inflammatory immune cells called monocytes respond to irritation in the arterial wall, then turn into macrophages whose job is to "eat" the offending lipids. The build-up of these cells can cause the plaque to rupture, leading to clots that can block blood flow. So for the new study, researchers at Michigan State University used this gathering of immune cells to their advantage. They highlighted the immune cells ?" and as such, the plaques ?" using nanoparticles made of carbon nanotubes.