托福阅读考试材料艾滋病病毒的超级抗体..docx

智课网TOEFL备考资料托福阅读考试材料：艾滋病病毒的超级抗体 托福 阅读主要考察学生理解文意的能力和短时间内提炼主题的能力，如果想要在 托福阅读 题中夺的高分，日常的练习时免不了的。下面，和小编一起来看看今天的 托福阅读材料 ： Scientists at the National Institutes of Health have identified long-sought and elusive broadly neutralizing antibodies to HIV in a pair of papers published in the July 9 issue of Science. These proteins produced by the immune system are crucial for creating a preventive vaccine, and could also have therapeutic uses developed in the coming years or decades. Variations in individuals immune systems can dramatically affect responses to infection—HIV is no exception. The result generally can be shown as a bell curve, with a group of people whose disease progresses rapidly, a broad middle segment who progress typically, and a small group of elite controllers whose immune systems are quite effective at containing HIV viral replication. The quest to figure out why has focused primarily on the adaptive immune system, because CD4+ and CD8+ T cells have a clearly demonstrated capacity to kill cells infected with HIV. But that response only arises some days, weeks and even months after a person has been exposed to HIV and the virus has integrated itself into cellular DNA, establishing lifelong infection. The adaptive immune response can only contain an established infection, it cannot prevent that infection from occurring at its onset. B cells are the first line of defense against infection. They attack at the initial exposure to a pathogen, and can prevent the establishment of infection—and HIV is no exception. But there are a number of reasons why it has proved difficult to identify their contribution to neutralizing the deadly virus. HIV transmission is not very efficient. Exposed persons may avoid infection for a variety of mechanical (barrier) and biological reasons, such as the viruss failure to penetrate to the surface of mucosal tissue or dendritic cell difficulties in latching onto the virus to carry it to a lymph node. So it is challenging to co