Understanding the immune system Self and Non-self The heart of
the immune system is the ability to distinguish between self and
non-self. Virtually every body cell carries molecules that
identify it as self. The body's immune defenses do not normally
attack tissues that carry a self-marker. When immune defenders
encounter cells or organisms carrying molecules that say
"foreign," the immune troops move quickly to eliminate the
intruders. Any substance capable of triggering an immune
response is called an antigen. Antigens can be a virus, a
bacterium, a fungus, or a parasite. An antigen announces its
foreignness by means of characteristic shapes called epitopes,
which protrude from its surface. Keeping Out Foreigners The
immune system stockpiles a tremendous arsenal of cells. In order
to have room to match millions of possible foreign invaders,
just a few of each type of antibody are stored. When an antigen
appears, those matched cells multiply into a full-scale army.
Antibodies belong to a family of large mole