Our immune system, a network of cells, tissues and organs, plays an important role in keeping us well. Simply put, this is the gatekeeper for anything that enters our bodies, determining whether it’s harmful or harmless, and acting accordingly. When bacteria or viruses enter the body, the body’s cells take action. Some produce antibodies, which are infection-fighting proteins. White blood cells attack bacteria directly. It’s these and other immune-system actions that can prevent a child from getting sick in the first place, or help him recover if he does become ill.