The complex defense network of cells attacks each cold virus—two to three a year—surveys the countless malignancies that threaten to become cancer, holds in check viruses like herpes that colonize huge swaths of the population, and confronts hundreds of millions of cases each year of food poisoning. Only recently have we begun to understand the pervasive role of our immune system in the brain, where damaged or outdated synapses get pruned by the organ’s own immune cells, allowing ongoing neurological health.
