a network of organs that contain cells which
recognize foreign substances and destroys them. All living organisms
are exposed to harmful substances and most can protect themselves in
several ways, either with physical barriers or chemicals that repel
and kill them. It protects vertebrates against viruses, bacteria,
fungi and other parasites. These viruses are called pathogens.
The immune system is known to help in the fight against cancer. A type
of white cell, which