The cells that are part of this defense system are white blood cells, or leukocytes, and are produced in huge quantities in the bone marrow. There are a wide variety of white blood cells, each with its own strengths and weaknesses.

The immune system displays several remarkable characteristics. It can distinguish between "self" and "non-self." It is able to remember previous experiences and react accordingly; thus, once you have had chicken pox, your immune system will prevent you from getting it again. The immune system displays both enormous diversity and extraordinary specificity; not only is it able to recognize many millions of distinctive non-self molecules, it can produce molecules and cells to match up with and counteract each one of them. And it has at its command a sophisticated array of weapons.