阅读理解　　The immune system is equal in complexity to the combined intricacies of the brain and nervous system. The success of the immune system in defending the body relies on a dynamic regulatory communications network consisting of millions and millions of cells. Organized into sets and subsets, these cells pass information back and forth like clouds of bees swarming around a hive. The result is a sensitive system of checks and balances that produces an immune response that is prompt, appropriate, effective and self-limiting. 　　At the heart of the immune system is the ability to distinguish between self and non-self. When immune defenders encounter cells or organisms carrying foreign or non-self molecules, the immune troops move quickly to eliminate the invaders. Virtually every body cell carries distinctive molecules that identify it as self. The body''s immune defenses do not normally attack tissues that carry a self marker. Rather, immune cells and other body cells coexist peaceably in a state known as self-tolerance. When a normally functioning immune system attacks a non-self molecule, the system has the ability to remember the specifics of the foreign body. Upon subsequent encounters with the same species of molecules, the immune system reacts accordingly. With the possible exception of antibodies passed during lactation ( 哺乳期 ), this so-called immune system memory is not inherited. Despite the occurrence of a virus in your family, your immune system must learn from experience with the many millions of distinctive non-self 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 self-markers ) 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, non-antigenic building blocks. An antigen announces its foreignness by means of intricate and characteristic shapes called epitopes( 抗原决定基), which stick out from its surface. Most antigens, even the simplest microbes, carry several different kinds of epitopes on their surface, some may even carry several hundred. Some epitopes 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 these diseases are caused by a person''s immune system actually attacking itself

【正确答案】 A

【答案解析】A项说法符合第2段第2句意思，其中reject=eliminate(排除，排斥)，alien相当于non-self：异己的；陌生的，为应选答案。 B项的all the substances(所有物质)不合句意，因句中的invaders(入侵者)是指 non-self cells or molecules(非自身细胞或分子)。C项说法貌似合理，实不科学，因免疫系统只能识别、消除侵入体内的异己细胞，而不能使身体不受细菌attack(侵袭)。D项只提identify and specify(识别和认定)，未说eliminate(消除)，故不全面。