Elie Metchnikoff

In 1882, Metchnikoff observed that certain mobile cells in starfish larvae could engulf and digest foreign particles, a process he named phagocytosis. He proposed that these phagocytic cells—the body's white blood cells in vertebrates—constituted the primary defense mechanism against invading microorganisms, founding the field of cellular immunology.

Metchnikoff shared the 1908 Nobel Prize in Physiology or Medicine with Paul Ehrlich, recognizing their complementary contributions to immunology: Metchnikoff for his theory of cellular immunity via phagocytes, and Ehrlich for his work on humoral immunity and the side-chain theory of antibody formation.

Metchnikoff made his initial phagocytosis observations using starfish larvae (specifically bipinnaria larvae), whose transparent bodies allowed him to watch mobile mesodermal cells gather around and engulf inserted foreign material such as rose thorns. He later confirmed similar behavior in Daphnia (water fleas) infected by fungal spores.

After leaving his position at the University of Odessa in Russia, Metchnikoff joined the Pasteur Institute in Paris in 1888 at the invitation of Louis Pasteur, where he remained for the rest of his career. At the Pasteur Institute he continued his immunological research and also developed theories about the role of intestinal bacteria in aging and health.