Immune - From a Latin word immunis meaning not subject to public service. It was the term used to describe legionaries with special skills that were exempt from routine duties.
Think of the human body as a huge society. There are some 10 trillion cells and another 100 trillion other microbes in our bodies (your personal count may vary). NPR's Robert Krulwich described the human race as just one big chain of microbe hotels. Within this zoo of benign and even symbiotic beneficial microbes the body's society is constantly fighting off invading species, usually without bothering to send out a general alarm.
The body has a two-tiered defensive system. The innate system is fast acting but indescriminate. My body's innate immune system, for example, tends to panic when it encounters certain pollens and animal dandriff. It released histamine to flood my respiratory system with protective mucus - When it doesn't need any protecting!! Hence my annoying bouts of hay fever
The adaptive system is a biological gem. It creates defenses to order and also makes long-lived memory cells that allow for a quick reaction if that particular invader ever returns.