The immune system is a complex network of cells, tissues and organs that work together to defend our bodies against infections and diseases. It is made up of two main parts - the innate immune system which provides an immediate response to any foreign substances or invaders, and the adaptive immune system which takes more time to develop a targeted defense against specific pathogens. The immune system plays a crucial role in keeping our bodies healthy by recognizing and eliminating germs, bacteria, viruses, and other foreign substances that can cause illness or disease.