How does tears defend the body against infection

They help to protect us against invading pathogens. You have beneficial bacteria growing on your skin, in your bowel and other places in the body (such as the mouth and the gut) that stop other harmful bacteria from taking over.

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the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.