In order for an organism to survive it must have a good defense.

This line of defense is its "Immunity" and it will help the body revisit foreign organisms.

Such foreign organisms are bacteria's or viruses, which come from the outside environment.

The immune system can also defend against cancer cell that can arise within the body.

Antigen is a foreign substance that enters the organism.

Pathogen is a living antigen (virus, bacteria) that enters the organism.

The immune system produces white blood cells which help fight these foreign invaders. This protection by the white blood cells can occur in three ways:
1. Engulfing (eating) invaders ex: phagocytes
2. Producing antibodies (chemicals which destroy or neutralize antigens) ex: lymphocytes
3. Marking antigens for attack and killing by other white blood cells.

Antibodies have a specific shape to destroy a specific antigen.