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

by John M. Barry

There are more aggressive defenses. Macrophages and “natural killer” cells—two kinds of white blood cells that seek and destroy all foreign invaders, unlike other elements of the immune system that attack only a specific threat—patrol the entire respiratory tract and lungs. Cells in the respiratory tract secrete enzymes that attack bacteria and some viruses (including influenza) or block them from attaching to tissue beneath the mucus, and these secretions also bring more white cells and antibacterial enzymes into a counterattack; if a virus is the invader, white blood cells also secrete interferon, which can block viral infection. All these defenses work so well that the lungs themselves, although directly exposed to outside air, are normally sterile. But when the lungs do become infected, other defenses, lethal and violent defenses, come into play. For the immune system is at its core a killing machine. It targets infecting organisms, attacks with a complex arsenal of weapons—some of them savage weapons—and neutralizes or kills the invader.