How do some of your cells swallow invading bacteria whole?

Some of your white blood cells defend you by literally eating invading bacteria whole. A cell called a phagocyte stretches part of its outer membrane around a germ, pulls it inside into a sealed pocket, and digests it there. This swallowing process is called phagocytosis, from Greek words meaning 'to eat' and 'cell.'

In 1882, the Russian zoologist Élie Metchnikoff spotted it in an unlikely place. He was studying starfish larvae and inserted tiny citrus thorns into them. Unusual cells clustered around the splinters. He realized that in animals with blood, white blood cells gather the same way at a site of inflammation, and guessed they might attack bacteria there too. A colleague suggested calling such a cell a 'phagocyte.' Metchnikoff later shared the 1908 Nobel Prize for his work on immunity and is remembered as the father of innate immunity.

Two cell types do most of this eating. Macrophages patrol almost every tissue in the body, engulfing pathogens and debris, and also help trigger the more specific, adaptive immune response by showing other immune cells what they have eaten. Neutrophils, the most abundant white blood cells, rush out of the bloodstream toward infections and kill what they catch; they are the main ingredient in pus, which is why it looks whitish-yellow.