millions of distinctive nonself molecules in the sea of microbes in which we live, learning necessitates producing the appropriate molecules and cells to match up with and counteract each non-self invader.
Any substance capable of stimulating an immune response is called an antigen. Tissues or cells from another individual (except an identical twin, whose cells carry identical selfmarkers) act as antigens; because the immune system recognizes transplanted tissues as foreign, it rejects them. The body will even reject nourishing proteins unless they are first broken down by the digestive system into their primary, nonantigenic building blocks. An antigen announces its foreignness by means of intricate and characteristic shapes called epitomes, which stick out from its surface. Most antigens, even the simplest microbes, carry several different kinds of epitomes on their surface, some may even carry several hundred. Some epitomes will be more effective than others at stimulating an immune response. Only in abnormal situations does the immune system wrongly identify self as non-self and execute a misdirected immune attack.
The result can be so-called autoimmune disease. The painful side effects of those diseases are caused by a person’s immune system actually attacking itself. (427 words)
1. We know from the passage that the immune system _______________
[A] is no less complicated than the nervous system.
[B] far exceeds the human
