So What is an Immune System?

I decided to research first about what the immune system actually is, and I found some information from Jodi Moore:

The immune system is made up of a complex network of cells and organs that protect the body from infection. Its main function is to keep harmful invaders like bacteria and viruses out of the body, or prevent them from negatively affecting our health.  The most important part of the immune system is the leukocytes, or white blood cells. There are B-cells, which recognize intruders, and T-cells, which are sent by the B-cells to kill germs that are harmful to our bodies. The amazing thing about these cells are that they can actually remember the best ways to fight off particular germs! This is why we usually don't get sick with diseases like chicken pox more than once.