Your immune cells defend you by literally eating invaders whole
When bacteria slip into a wound, some of the first responders simply swallow them. The cell wraps its membrane around the intruder, seals it inside a bubble and digests it. This cell eating, called phagocytosis, was the first immune mechanism ever discovered, and it is so ancient that even invertebrates rely on it.
The name joins Greek words for eating and cell. It applies when a cell engulfs something at least half a micrometre across, enclosing it in a compartment called a phagosome where it gets broken down. Targets include bacteria, dead cells and tiny mineral grains. Some single-celled organisms feed this way, a strategy biologists call phagotrophy, as opposed to absorbing nutrients directly.
The discovery unfolded over decades. Albert von Kölliker described an amoeba-like protist engulfing tiny organisms in 1849. In 1862 Ernst Haeckel showed that blood cells from a sea slug would swallow particles of Indian ink, the first direct evidence that immune cells do it too. William Osler noticed the process in 1876, and Élie Metchnikoff studied it closely and gave it its name in the early 1880s.
Almost any cell can manage it, but some make it their career. These professional phagocytes include neutrophils, macrophages, monocytes and dendritic cells, and even the osteoclasts that remodel bone. Neutrophils patrol the blood and rush into infected tissue in large numbers, then kill what they ingest by dumping granules of enzymes and antimicrobial proteins into the phagosome along with a burst of reactive oxygen. Macrophages settle in tissues as long-lived sentinels and can keep eating by making fresh digestive compartments. Dendritic cells swallow microbes for another reason, chopping them up to show fragments to the adaptive immune system.
To decide what to eat, phagocytes use surface receptors. Some detect tags the body sticks onto invaders, such as antibodies or complement proteins; others recognise foreign sugars like mannose directly. The route in can differ: antibody-coated targets are grabbed by a cup of membrane that reaches out around them, while complement-coated ones sink into the surface with no protrusions at all.
Source: Phagocytosis
