Did you know that the immune system is called the system of “self” because it possesses the remarkable ability to tell self from nonself, friend from foe? It recognizes and destroys cancer cells, transplanted tissue cells, and a wide range of organisms, from the minute picornavirus, which are so small that more than a million lined up would fit in a space of an inch, to some parasites that are visible to the naked eye.
At the same time, it usually respects the body’s own tissues, varied as they are. Sometimes, the difference between self and foreign is slight, a matter of only a molecule or two, as in the distinction between a normal cell and a cancerous one.
It knows what to respect and what to reject!
Amazing isn’t it?