After even more stages, macrophages (large yellow cells) may begin appearing at the borders. Macrophages are the body's natural defenses against intruders like nanobots. If the nanobot is touched by a macrophage, it will be destroyed and a new one will be reinjected at the start. Your defense is to activate your shields. Any macrophage touching the shields will be destroyed and a new phage will be called to the rescue.

Shields On 

Beware: Macrophages are not limited to neurons and can travel in any direction. The longer you remain in the brain, the more of them will appear. Eventually, there will be too many of them to avoid and you will likely find your score is decreasing after each stage. Your best option at that point is to return to the scoring page and select "New Game" from the menu.