The immune system normally protects the body against foreign materials, such as viruses and bacteria.

免疫系统通常保护身体不受异物的影响，如病毒和细菌。

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The immune system loses its ability to tell the difference between foreign materials and its own cells.

免疫系统不能识别外来物质和自身细胞的区别。

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Because it has the ability to absorb foreign materials, it will yellow over time if it is not properly maintained.

因为它有吸收外界物质的能力，如果不妥善保管的话就会泛黄。

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The immune system loses its ability to tell the difference between foreign materials and its own cells.

VOA: special.2010.06.15

Then, in May, the Pentagon's Defence Contract Management Agency announced an inquiry into allegations that the company was violating the (aptly-named) Berry amendment, which has prohibited the use of foreign materials in products bought by the Department of Defence for more than 50 years.

ECONOMIST: Well, hardly

First, the high U.S. corporate demand for foreign labor and raw materials alone causes a trade imbalance.

FORBES: Is 95% of the Trade Imbalance Legal Tax Evasion?

More so than retailers and service providers, manufacturing companies have foreign suppliers and must import raw materials from around the world.

FORBES: Manufacturing Takes Its Time, High A/P Days