Systematic Immunity is a challenging precision platformer set inside the human body...and not a particularly healthy one either. You control the body's white blood cells to fight off an infection that is being spread throughout the body's organs by a particularly nasty germ. In each level this germ will produce 15 smaller germs that have to be taken out by your white blood cells.
The white cells are spawned in groups of 20, with one "leader cell" who is controlled by the player. Unfortunately the other cells aren't very bright, so they just blindly follow the actions of their leader. Throughout the game you have to run, jump, roll, float, bounce and teleport your way through the inhospitable tissue of your host human, and all the while keeping your group off cells alive! Many will die...in fact hundreds and thousands will die. But luckily they are plentiful and very persistent.