The Great Influenza: The Epic Story of the Deadliest Plague in History

by John M. Barry

The immune system begins its defense far in advance of the lungs, with enzymes in saliva that destroy some pathogens (including HIV, which makes its home in most bodily fluids, but not in saliva, where enzymes kill it). Then it raises physical obstacles, such as nasal hairs that filter out large particles and sharp turns in the throat that force inhaled air to collide with the sides of breathing passageways. Mucus lines these passageways and traps organisms and irritants. Underneath the layer of mucus lies a blanket of “epithelial cells,” and from their surfaces extend “cilia,” akin to tiny hairs which, like tiny oars, sweep upward continuously at from 1,000 to 1,500 beats a minute. This sweeping motion moves foreign organisms away from places they can lodge and launch an infection, and up to the larynx. If something does gain a foothold in the upper respiratory tract, the body first tries to flush it out with more fluid—hence the typical runny nose—and then expel it with coughs and sneezes. These defenses are as physical as raising an arm to block a punch and do no damage to the lungs. Even if the body overreacts, this usually does no serious harm, although an increased volume of mucus blocks air passages and makes breathing more difficult. (In allergies these same symptoms occur because the immune system does overreact.)