Answer:
Skin, tears, and mucus are part of the first line of defense in fighting infection. They help to protect us against invading pathogens. The skin creates a barrier against pathogens using hard proteins on its surface. The second line of defense is nonspecific resistance that destroys invaders in a generalized way without targeting specific individuals: Phagocytic cells ingest and destroy all microbes that pass into body tissues. For example, macrophages are cells derived from monocytes (a type of white blood cell). The third line of defense is specific resistance. This system relies on antigens, which are specific substances found in foreign microbes. Most antigens are proteins that serve as the stimulus to produce an immune response. The activation of T-cells by a specific antigen is called cell-mediated immunity.
