Which statement describes many of the structures involved in the first line of defense of the immune system?

Many first-line defenses, such as the skin and the linings of the respiratory and digestive tracts, form barriers where the body meets the outside world. They help keep pathogens from entering the body, so they have contact with the external environment. B-cells and T-cells are involved in later immune responses, not produced by these barriers in response to a pathogen.

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