Regulatory T cells police the immune system, making sure that killer T cells coming back from battling pathogens transition into a resting state, but still prepared to fight if the pathogen returns.

Certain immune cells pick up food particles, chop them into fragments and present them to the immune system's T cells, instructing those cells to remain unresponsive to the harmless intruder.

The immune system is the body’s tool for preventing or limiting infection. Its complex network of cells, organs, proteins, and tissues enables the immune system to defend the body from pathogens.