Any substance capable of stimulating an immune response is called an antigen (抗原). Antigens are not to be confused with allergens (过敏原),which are most often harmless substances that provoke the immune system to set off the inappropriate and harmful response known as allergy. An antigen can be a virus, a bacterium, or even a portion or product of one of these organisms. Tissues or cells from another individual (except an identical win, whose cells carry identical self makers 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 epitopes (抗原表位), which protrude (突出) 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 nonself and execute a misdirected immune attack.
36 What is used to describe the communication network consisting of cells in the immune system?
A The immune systems memory.
B Bees flying around a hive.
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