The Library of Congress · Public domain

Discovered phagocytosis — the process by which certain white blood cells engulf and destroy pathogens — earning him recognition as the father of innate immunity and a shared 1908 Nobel Prize with Paul Ehrlich. Born in the Russian Empire to a Ukrainian-Jewish mother, he was baptized Russian Orthodox as a child and later became an atheist; his advocacy for lactic-acid bacteria as a health measure seeded the modern concept of probiotics.