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  3. Immune Functions The immune system is equal in complexity to the combined intricacies of the brain and nervous system. The s

Immune Functions The immune system is equal in complexity to the combined intricacies of the brain and nervous system. The s

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问题                       Immune Functions
    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 communication network consisting of millions and millions of cells. Organized into sets and subsets, these cells pass information back and forth like clouds of bees flying around a hive(蜂巢). 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 nonself. When immune defenders encounter cells or organisms carrying foreign or nonself molecules, the immune troops move quickly to eliminate the intruders(入侵者).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 nonself 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(抗体) passed during lactation (授乳期), 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 nonself molecules in the sen of microbes(微生物) in which we live. Learning entails producing the appropriate molecules and cells to match up with and counteract each nonself invader.
    Any substance capable of triggering 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 also 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 epitopes on their surface; some may even carry several hundreds. 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 nonself and execute a misdirected immune attack.
Which of the following best expresses the main idea of this passage?______
选项 A、An antigen is any substance that triggers an immune response.
B、One of the immunes system’s primary functions is the allergic response.
C、The human body is an appropriate habitat for microbes.
D、The basic function of the immune system is to distinguish between self and nonself.
答案D
解析 本题有一定难度, 需要通读全文, 吃透原文句意。文章第一段主要讨论了免疫系统的基本信息交流问题, 第二段集中讨论了免疫系统的核心作用即识别自我和非我物质, 第三段主要讨论了非我物质抗原和过敏原问题, 所以可以断定文章的中心是第二段的内容, 回来看选项, 很明显, D项符合原文主要意思, 是答案。
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