The immune system is a complex biological system that both identifies and elminates foreign bodies.

To be able to do the IS must have advanced learning and recognition.

For example it has the ability to learn about a foreign body (antigen). After this it then has the ability to recognise antigens (through antibodies) it has encountered before and remember them even though many years have passed since first encountering them.  This is how immunisation works, for example the IS is allowed to learn about Polio and develop a response, so when the body encounters it many years down the road it knows how to react.