Pathogens that get past the body’s outer protections, especially the skin, will find a hard time of it inside, where a vast array of immune functions muster to battle the invaders. Bones, lymph nodes, spleen, thymus, and various nooks and crannies suddenly spew forth antibodies—microbe-killing cells and chemicals—along with repair cells and chemicals in great profusion. By one count, the body contains 300 or more different types of immune cells. Their job description is simple: “identify anything that is in the body that shouldn’t be there and, if necessary, kill it” (201). 

New pathogens and mutating older ones evolve all the time, trying to defeat the immune system, which thus has an endless task. The immune system also must remove body cells that go bad, as happens in cancer. Occasionally, mistakes happen, and an autoimmune disease will erupt—arthritis, Crohn’s disease, multiple sclerosis, and many others. Five percent of humans suffer from such problems. 

Five types of white blood cells anchor the immune system; of these, lymphocytes are the main players, in two types: B and T. T-cells remember previous invaders, marshal other cells to do battle, and instruct B-cells which antibodies to produce. Antibodies latch onto invaders, disabling and tagging them for attack by other immune elements.