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  3. An often-used, but valuable analogy compares the immune system with an army. The defending troops are the white blood cells call

An often-used, but valuable analogy compares the immune system with an army. The defending troops are the white blood cells call

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问题     An often-used, but valuable analogy compares the immune system with an army. The defending troops are the white blood cells called lymphocytes, born in the bone marrow, billeted in the lymph nodes and spleen, and on exercise in the blood and lymph systems. A body can muster some 200,000,000 cells, making the immune system comparable in mass to the liver or brain.
    The lymphocytes are called to action when the enemy makes itself known. They attack anything foreign. Their job is to recognize the enemy for what it is, and then destroy it. One of the key features of the immune system is its specificity. Inoculation with smallpox provokes an attack on any smallpox virus, but on nothing else. This specificity of response depends on the lymphocyte’s ability to identify the enemy correctly by the molecules on its surface, called antigens.
    An antigen is an enemy uniform. It can be a protein on the surface of a cold virus, or it can be a protein on the surface of a pollen grain, in which case the immune response takes the form of an allergy. An antigen can also be a protein on the surface of a transplanted organ, in which case the immune response "rejects" the transplant. Organs can therefore be transplanted only between closely related people—in whom the antigens are the same—or into people treated with a drug that suppresses the immune system, such as cyclosporin.
    In the 1940s, an Australian immunologist, Sir Frank Macfarlane Burnet, proposed a theory that helped explain how lymphocytes recognize and are activated by specific foreign antigens. The clonal selection theory as it was called, suggested that the innate immune system was not a homogeneous mass of more or less identical lymphocytes, but rather was made up of millions of different families called clones. The members of each clone carry on their surfaces a receptor that is capable of identifying and binding to just one foreign antigen(or a part of it called the determinant).
    Thus, when a foreign body carrying that antigen appears, the antigen binds to the receptor of only those lymphocyte clones which are capable of recognizing it. Once the antigen binds to the receptor, it stimulates the lymphocyte to divide, which generates more identical copies of itself. These clone members then attack the foreign entity which carries the antigen.
    This implies that the immune system works on a "ready-made" basis. A person’s immune system inherits the knowledge of all foreign antigens to which it might be exposed. The sum of this inheritance increases as new threats are met.
The author’s primary purpose in the passage is to do which of the following?
选项 A、Demonstrate the inadequacy of an analogy.
B、Advocate a method to strengthen the immune system.
C、Compare the immune system to the brain.
D、Clarify the workings of the body’s defense system.
答案D
解析 纵观全文，文中首句就提到一个经常使用但具有价值的比喻是把免疫系统比作军队。后面通过讲解免疫系统中的淋巴细胞的生成、功能来阐明免疫系统是如何保护人体的。最后又说这意味着免疫系统是在“现成”的基础上工作的。据此可知，本文作者的主要目的是阐明免疫系统的工作方式，D项正确。
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