The immune system is made up of special cells and chemicals that fight infection. The white blood cells that make up the immune system are made in the bone marrow. These cells move through blood and tissue. Every time a microbe (germ) is overcome, the immune system remembers that microbe. If the body comes in contact with that microbe again, it will be defeated quickly.
The immune system also produces proteins called antibodies that can help neutralise infection or the toxins that some germs produce.
Information courtesy of The Better Health Channel.