The Song of the Cell: An Exploration of Medicine and the New Human

by Siddhartha Mukherjee

An astonishing feature of this early immune response is that its cells, neutrophils and macrophages among them, are intrinsically armed with receptors that recognize proteins (and other chemicals) found on the surface or the interior of some bacterial cells and viruses. Pause for a moment to consider that fact. We—multicellular animals—have been at war with microbes for such a long time in evolutionary history that, like ancient, conjoined enemies, we’ve been defined by each other. We are in a lockstep dance. Our first-responder immune cells carry pattern-recognition receptors that are inherently designed to latch on to molecules found in microbial cells or injured cells that are not specific to a specific pathogen (Streptococcus, say) but are broadly present in all bacteria and viruses. Some receptors recognize a protein found in bacterial cell walls but not in animal cell membranes. Some bind to a protein found uniquely in the swimming tails of some bacteria. Yet others sense signals sent out by cells that have been infected by viruses. Speaking generally, these receptors come in two classes: those that recognize “damage-associated molecular patterns” (substances released upon cellular damage) and those that sense “pathogen-associated molecular patterns” (components of microbial cells). In short, they sniff around the body looking for patterns of injury and infection—substances that signal invasion and pathogenicity. When a neutrophil or a macrophage meets a bacterial ce...