Passage Five
The immune system is the body’s defender. It identifies，tracks down，and destroys troublemakers before they can hurt the body. Those troublemakers maybe，for example，bacteria from a cut or splinter，a measles germ，a cold bug，or even a cancer cell.
These invaders try to take over our tissues and feed off our bodies’nutrients. If they succeed，we become sick or，sometimes，even die. It’s the job of our immune systems to destroy these invaders before they destroy us.
Say you picked up a flu virus last week. Perhaps it entered your body through a cut，from a drinking glass，or from the air you breathe. Of course，you didn’t feel it，but the virus made its way into your bloodstream. As soon as it entered your body，it began to reproduce. Viruses have only one goal：to take over your cells. Once inside your body，viruses try to enter cells and disrupt their normal work. If left alone，these viruses would hurt so many cells that you would weaken，or worse，get a serious disease.
But this flu virus should not be so complacent as it seems to be. As it reproduces in your bloodstream，the virus is met by a certain kind of white blood cell，the lymphocytes. The lymphocytes are the foot soldiers that keep you alive.
The number of lymphocytes in your body is hard to imagine. Thousands of them could fit in the period at the end of this sentence. Your body holds about a trillion—that’s 1，000，000，000，000 of them，or about 3，000 in every drop of blood. Since you began reading this sentence，over 800，000 of them have been created and destroyed.
Some of these lymphocytes pass through a small walnut-sized organ called the thymus. The thymus is the base of the neck. Here，special hormones turn lymphocytes into fighting cells，called T cells.
T cells have one terrific talent：they can tell the difference between friend and foe；what should be in our bodies and what shouldn’t. They do not affect the body’s healthy cells. Yet they attack everything that is foreign to our bodies，such germs，transplants，and even our own cells which have become abnormal，as in the case of cancer.
21. What is the function of our immune system？
A. To take over our tissues and feed off our bodies’nutrients.
B. To prevent bacteria or germs from entering the body.
C. To destroy the troublemakers in order that they might not hurt the body.
D. To fight against T cells.
22. What does the word“lymphocytes”in Para.4 mean？
A. Foot soldiers. B. Flu virus. C. White blood cells. D. Trouble makers.
23. The last sentence in Para.5“Since you began reading this sentence，over 800，000 of them have been created and destroyed”implies that .
A. this sentence contributes to the increase of the number of lymphocytes in your body
B. you would not realize that lymphocytes reproduce themselves rapidly but for this sentence
C. because you read this sentence，a large number of lymphocytes have been consumed
D. the number of lymphocytes in your body is extremely large
24. Which of the following questions can NOT be detected by T cells？
A. Thymus. B. Transplants. C. Cancer cells. D. Healthy cells.
25. Which of the following questions best summaries the main idea of the whole passage？
A. How does the immune system attack the flu virus？
B. How does the immune system function in our body？
C. How do the lymphocytes work in our body？
D. What is the so-called immune system？

参考答案：21-25 CCDAB