Final answer:
The human body's first line of defense includes physical and chemical barriers such as the skin, tears, mucus, and cilia.
Explanation:
The human body's first line of defense against pathogens includes physical and chemical barriers such as the skin. Pathogens are killed or inactivated on the skin by desiccation and the skin's acidity. Other defenses like tears, mucus, and cilia in the respiratory tract also play crucial roles in protecting the body.
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