Cells are a great template for building a game, I've had similar ideas about making games where immune systems gather together to defeat invaders.
When I started playing I found myself getting defeated over and over while trying to use immune cells but quickly found that a single enzyme on the corner bend and an immune cell near the exit was enough.
However, once I started placing many cells I found that only one or two can fire at once despite there being many available. Not sure if this was intentional or not.
When selecting an immune cell in full screen mode it's easy to accidentally place one on the grid behind the button.
All in all it's a pretty nice tower defense template.