The immune system is a complicated mechanism whose principle actors are organs, cells, chemicals and a vast collection of proteins tuned by billions of years of evolution to work in a harmonious manner for the good of the whole. Its purpose is to keep us alive in a world where visible and invisible predators intending to usurp our bodies attack us on a daily basis.
By virtue of an amazing communication network, millions of cells gather, exchange information, become activated and respond to bacteria, fungi, parasites and viruses preventing them from causing infection.
A malfunctioning immune system, when immune cells attack "self," results in numerous auto-immune diseases, the "God only knows" diseases of sixty years ago.