单选题 When my son started going to "school" full time in February, I readied myself for immunological battle. Day-care kids get sicker than children who stay at home, and I knew mine" would, too. But other parents assured me that by kindergarten he"d be the healthiest kid in class. Last week parenting message boards lit up when a University of California, Berkeley, researcher presented unpublished data showing that children who attend playgroups or day care have a 30 percent lower risk of developing childhood leukemia than kids who don"t, possibly because they are exposed to more infections early in life.
The human immune system is an elegant mix of two parts—a built-in, or innate, system and an acquired one. The innate system has already read the manual on generic germs. The acquired system, by contrast, is a bookworm, reading on the go,, learning with every new microbial visitor and growing wiser as it ages. Together, the two systems assess the foods we eat, the particles we breathe, the bacteria we touch, then determine whether or not to attack.
Can a young immune system handle so much new information? Research published over the past decade is reassuring. Scientists at the University of Arizona found that 2-year-olds who attend day care in the first six months of life have almost twice as many colds as stay-at-home kids. But they have a third fewer colds between the ages of 6 and 11. By 13, there"s no difference in the groups, suggesting that the kids" immune systems catch up with each other. Several studies have found that children who go to day care early in life are also less likely to develop asthma.
The Arizona scientists discovered that high-risk children who start day care before 3 months old have lower levels of immunoglobutin E—a marker of allergic susceptibility connected to asthma-than non-day-care kids. Those levels remain low for the first three years of life. Anne Wright, the study"s lead author, says this doesn"t necessarily mean that kids benefit from being sick more often. She believes the findings support the "hygiene hypothesis," which suggests that simply being exposed to more microbes—which run rampant at day care—educates the immune system, making it less likely to launch unwarranted warfare.
All this is good to know. But I had to ask the experts: why am I getting: so sick? "Because you live with the source," says Liu. And I hug and kiss him a lot, too, so I"m probably getting a big dose of germs. It"s also possible that my immune system"s memory has faded a bit, making old harmless viruses look new and dangerous. Or I may be meeting bugs my immune system has never seen before. The most comforting words I heard were from Columbia University pediatrician Philip L. Graham Ⅲ, who told me that pediatricians get horribly sick during their first year of treating patients. After that, they"re immunological powerhouses.

单选题 The findings of the research at the University of California, Berkeley show that ______

单选题 The two parts of our immune system have different functions in that ______

单选题 From the third paragraph we learn that ______

单选题 An "unwarranted warfare" (Para.4) for the immune system is one in which ______

单选题 From Doctor Philip L. Graham Ⅲ, the author has got the reassuring message that ______