任何食物对身体而言都是外来物质。
免疫系统不能识别外来物质和自身细胞的区别。
The immune system loses its ability to tell the difference between foreign materials and its own cells.
它还含有脓，血液，纤维，细菌产物或其他外来物质。
It can also contain pus, blood, fibrin, bacterial products or other foreign matter.
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