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IV. 閱讀測驗 10%     They lie buried- their long, tentaclelike arms outstretched—in all the tissues of our bodies that interact withthe environment. In the lining of our nose and lungs, lest we inhale the influenza virus in a crowded subway car.In our gastrointestinal tract, to alert our immune system if we swallow a dose of salmonella bacteria. And mostimportantly, in our skin, they lie in wait as stealthy sentinels should microbes breach the leathery fortress of ourepidermis.    They are dendritic cells, a class of white blood cells that encompasses some of the least understood butmost fascinating actors in the immune system. Over the past several years, researchers have begun to unravelthe mysteries of how dendritic cells educate the immune system about what belongs in the body and what isforeign and potentially dangerous. Intriguingly, they have found that dendritic cells initiate and control theoverall immune response. For instance, the cells are crucial for establishing immunological “memory,” which isthe basis of all vaccines. Indeed, physicians, including those at a number of biotechnology companies, aretaking advantage of the role that dendritic cells play in immunization by “vaccinating” cancer patients withdendritic cells loaded with bits of their own tumors to activate their immune system against their cancer.Dendritic cells are also responsible for the phenomenon of immune tolerance, the process through which theimmune system learns not to attack other components of the body.    Dendritic cells are relatively scarce: they constitute only 0.2 percent of white blood cells in the blood andare present in even smaller proportions in tissues such as skin. In part because of their rarity, their true functioneluded scientists for nearly a century after they were first identified in 1868 by German anatomist PaulLangerhans, who mistook them for nerve endings in the skin.    In 1973 Ralph M. Steinman of the Rockefeller University rediscovered the cells in mouse spleens andrecognized that they are part of the immune system. The cells were unusually potent in stimulating immunity inexperimental animals. He renamed the cells “dendritic” because of their spiky arms, or dendrites, although thesubset of dendritic cells that occur in the epidermis layer of the skin are still commonly called Langerhans cells.    There are several subsets of dendritic cells, which arise from precursors that circulate in the blood and thentake up residence in immature form in the skin, mucous membranes, and organs such as the lungs and spleen.Immature dendritic cells are endowed with a wealth of mechanisms for capturing invading microbes: they reelin invaders using suction cup-like receptors on their surfaces, they take microscopic sips of the fluid surroundthem, and they suck in viruses or bacteria by engulfing them in sacks known as vacuoles. Once they devourforeign objects, the immature cells chop them into fragments (antigens) that can be recognized by the rest of theimmune system.    Dendritic cells are very efficient at capturing and presenting antigens: they can pick up antigens that occur inonly minute concentrations. As they process antigens for presentation, they travel to the spleen through theblood or to lymph nodes through a clear fluid known as lymph. Once at their destinations, the cells completetheir maturation and present their antigen-laden molecules to naïve helper T cells, those that have neverencountered antigens before. Dendritic cells are the only cells that can educate naïve helper T cells to recognizean antigen as foreign or dangerous. This unique ability appears to derive from costimulatory molecules on theirsurfaces that can bind to corresponding receptors on the T cells.    Once educated, the helper T cells go on to prompt so-called B cells to produce antibodies that bind to andinactivate the antigen. The dendritic cells and helper cells also activate killer T cells, which can destroy cellsinfected by microbes. Some of the cells that have been educated by dendritic cells become “memory” cells thatremain in the body for years—to combat the invader in case it ever returns.    Activating naive helper T cells is the basis of vaccines for everything from pneumonia to tetanus toinfluenza. Scientists are now turning the new knowledge of the role that dendritic cells play in immunity againstmicrobes and their toxins into a strategy to fight cancer.    Cancer cells are abnormal and as such are thought to generate molecules that healthy cells don’t. Ifresearchers could devise drugs and vaccines that exclusively targeted those aberrant molecules, they couldcombat cancer more effectively while leaving normal cells and tissues alone—thereby eliminating some of thepernicious side effects of chemotherapy and radiation, such as hair loss, nausea and weakening of the immunesystem caused by destruction of the bone marrow.    Such trials generally employ vaccines made of dendritic cells precursors that have been isolated from cancerpatients and grown in the laboratory together with tumor antigens. During this process, the dendritic cells pickup the antigens, chop them up and present them on their surfaces. When injected back into the patients, theantigen-loaded dendritic cells are expected to ramp up patients’ immune response against their own tumors.     Several researchers fear that such vaccines might induce patients’ immune systems to attack healthy tissueby mistake. In addition, tailoring a dendritic cell vaccine to fight a particular patient’s tumors might not beeconomically feasible. But many scientists are working to circumvent the costly and time-consuming steps ofisolating cells from patients and manipulating them in the laboratory for reinjection.    As we learn more about the molecules that control dendritic cells, we will find ways to harness theirtherapeutic potential. The increasing number of scientists and corporations working on dendritic cells portendsthat we will soon be able to maximize the biological power of these cells to treat and prevent the diseases thatplague humankind.[!--empirenews.page--]
41. Which is the best title for this article?
(A) Dendritic Cells- An Elixir for All
(B) The Long Arm of the Immune System
(C)An Introduction to Cancer Cures
(D) The Mechanisms for Creating Vaccines

參考答案

答案：B
難度：適中0.5
統計：A(0),B(0),C(0),D(0),E(0)

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