Skin and Mucus membranes Skin is an impenetrable barrier to viruses and bacteria if it is not broken. It is reinforced by chemical defenses. Oil and sweat give the skin a pH of 3-5. Bacteria that live on the skin help by releasing acids and wastes to kill off unwanted microorganisms. Lysozyme is found in tears and saliva which can destroy the cell walls of most bacteria.
Gastric juices kill off bacteria in food.
Mucus captures bacteria in the nose and throat cavities of the body. Phagocytes and Natural Killer Cells Phagocytes are cells that eat other cells or objects found in the body. Macrophages a type of phagocyte that eats bacteria, viruses, and the debris of damaged cells. Neutrophils are another type of white blood cell capable of becoming phagocytotic. Natural Killer Cells destroy the bodies own infected cells, especially those containing viruses.