Which statement describes many of the structures involved in the first line of defense of the immune system? They have contact with the external environment. They are internal and contact the pathogen through blood. They produce B-cells in response to a pathogen. They produce T-cells in response to a pathogen.
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The statement "They have contact with the external environment" describes many of the structures involved in the first line of defense of the immune system. The first line of defense includes physical and chemical barriers, such as the skin and mucous membranes, that act as the body's initial protection against pathogens and have direct contact with the external environment.