Because bacteria are outside of cells and when antibodies bind to them they can neutralize them.

因为细菌生存在细胞外,抗体结合细菌并中和掉它们

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They can't always neutralize viruses because the viruses are predominantly inside cells.

抗体不是总能中和病毒,因为病毒主要在细胞中

耶鲁公开课 - 生物医学工程探索课程节选