Answer: The immune system
Explanation:
The immune system is a complex network of proteins and cells that defends the body against infection or any invasion. The human defense system in the body is actually made up of entire organs and vessel systems like the lymph vessels. The immune system is made up of organs that control the production and maturation of certain defense cells.
Initially, all living things are subjected to attack from disease causing agents. Even bacteria, so small that more than a million could fit on the head of a pin, have systems to defend against infection by viruses. This kind of protection gets more sophisticated as organisms become more complex.
Take for example - you touching something sharp and instantly moving your hand away.
Sensory neurons in your fingers detect the external stimuli of the sharp object and send a nerve impulse to the spinal cord (does not reach brain) and is sent back to a motor neurone - attached to an effector that moves the muscle and moves the hand away. This is a rapid reflex action and involuntary.
