15．HIV is a tricky virus that escapes typical immune responses．During a successful immune system response to a foreign body，white blood cells produce antibodies that target the foreign body．These antibodies then mark the foreign body for destruction by other immune cells．For the most part，HIV escapes these immune defenses，but rare individuals develop antibodies that effectively block the virus．Researchers are now showing that these antibodies can also act as treatments in other HIV patients．
HIV has several ways of escaping the immune response．Unlike most viruses，HIV specifically attacks a type of white blood cell that is critical to our immune system．During reproduction，the HIV virus also picks up many new mutations （變異），which often change it enough that any antibodies produced earlier during the infection no longer recognize it．
These rare antibodies can also keep the virus from infecting new cells，which could make them an effective treatment．In animal studies，injection of low concentrations of these antibodies could act as a vaccine （疫苗） and provide protection against infection．Injections can even control active infections when combined with additional antibodies that target other molecules （分子） on the HIV surface．These findings led to humans phase 1clinical trials to evaluate a specific antibody that targets HIV．This study revealed that a single injection of the antibody typically reduced the presence of HIV in the blood of patients who have viruses that were sensitive to the antibody．
After performing test-tube studies showing the effectiveness of the antibody，the researchers evaluated its safety in humans in a small trial．The study included two groups of participants：14uninfected individuals and 19individuals with an HIV-1infection．The researchers then tracked the clearance of the antibody by following its levels in the participants'blood．Consistent with previous studies，HIV-1infected individuals exhibited faster clearance of the antibody，with a half-life of 12.8days compared to 24.0days for uninfected participants．Despite the fast clearance，however，the antibody appeared to be effective．
Thirteen HIV-1infected participants with the highest levels of virus received the highest dose （劑量） of the antibody．Eleven of them showed a rapid decline in HIV levels．Tracking the infection through the first weeks after treatment revealed the evolution of multiple viruses that were no longer affected by the antibody．However，these new variants （變體） generally remained sensitive to antibodies targeting other virus surface molecules．
Overall，this investigation shows the safety of injections of antibody 10-1074in humans．Thus，researchers may be able to build a cocktail of antibodies that effectively block active HIV infections．
59．HIV escapes immune responses byC．
A．removing the foreign body
B．identifying the immune system
C．a(chǎn)ttacking a certain white blood cell
D．reproducing some effective antibodies
60．What does the underlined word"it"in Paragraph 2refer to？C
A．The immune system．
B．A type of white blood cell．
C．The HIV virus．
D．An antibody．
61．The purpose of the further study of antibodies is toD．
A．introduce a vaccine protecting humans against infection
B．confirm the reduction of HIV in the blood of the patients
C．track a faster clearance of antibodies in uninfected subjects
D．prove the safety and effectiveness of antibodies in humans
62．What can we infer from the passage？B
A．Antibodies remain ineffective against most HIV viruses．
B．Researchers feel optimistic about a new cure for HIV．
C．Researchers get contradictory findings from a series of studies．
D．Antibodies can evolve with HIV viruses from infected patients．
