Physical barriers block pathogens and foreign substances from entering the body. Unpunctured skin is the most extensive and obvious wall; note that the bacteria and other microbes in figure 34.3 are entering the boy’s body at his skinned knee. Other physical barriers include mucus that traps inhaled dust particles in the nose; wax in the ears; tears that wash irritants from the eyes and contain an antimicrobial substance called lysozyme; and cilia that sweep bacteria out of the respiratory system. In addition, a bath of strong acid destroys most microbes that reach the stomach.
An often underappreciated component of this first line of defense is the body’s normal microbiota. Resident microbes on the skin, in the gut, and elsewhere help prevent colonization by pathogens. Moreover, studies of lab animals reared in special microbe-free environments show that the immune system does not develop correctly without stimulation from our microscopic companions.