Immunity is the power to resist against diseases and different foreign bodies like antigens. It is the system to fight with foreign antigen as well as the ability to develop self supporting mechanism. The human body is a very vulnerable organism, except for the very strong line of defense that the immune system has thrown around it. Without the natural defense system provided by the immune system, the bacteria, microbes, viruses, toxins, parasites, etc., would reduce the body to its chemical components in a few days.
That is what happens when the body dies and the immune system is no longer there. The human immune system works 24X7, though its work never comes to the limelight.
Although we inhale and eat thousands of germs everyday, the immune system prevents them from causing diseases. When a germ breaks through the immune system occasionally, we wind up with a disease. Once the immune system learns about these germs, it will fight them, and we get over the malady.