There are many mechanisms by which the body responds to foreign invaders. One of these involves the T-cells of the immune system, which have a number of different proteins on their surface called 'checkpoint proteins.' These checkpoint proteins bind to proteins on the surface of other cells and can result in either stimulation or suppression of T-cell activity. Normally, surface proteins on foreign or invading cells will produce a stimulation of T-cell activity against these cells, while T-cell suppression is a built-in mechanism to prevent the immune system from attacking the body's own normal cells.