Answer: "The immune system is a host defense system comprising many biological structures and processes within an organism that protects against disease." an article on Wikipedia says.
Explanation: Personally, I'd say it is your bodies way of fighting off the microscopic dangers of the world like parasites, fungi, bacteria, and viruses . It also learns how to fight off these things when you get sick with them or when you get given a vaccine because your immune system will have a metaphorical scar that it remembers and will use when the contagion enters your body.