The skin, mucous membranes, hair follicles, and sweat glands are all protectors. Each of these factors restrict the growth of, and protect the body from, microorganisms. The areas of the body which have openings to the outside of the body, such as the nasal passages, mouth, lungs, and digestive system, are lined with mucous membranes. The mucous traps many microorganisms and prevents them from entering further into the body. Sweat washes microorganisms from the pores and skin surface.
Tears wash away foreign substances from the eyes.
Nonspecific defenses rely on phagocytosis to fight infection. This is the process of leukocytes consuming foreign substances or dead tissue rather than healthy body cells. The specific defenses of the immune system can discriminate among antigens by forming specific proteins called antibodies . Antibodies are specialized to react with antigens, and protect the body by destroying the invader. They cannot kill foreign organisms on their own, but they can initiate the killing of such organisms by activating phagocytes, and natural killer cells.