Ilya Ilyich Mechnikov (Russian:Илья́ Ильи́ч Ме́чников, also written as Élie Metchnikoff) (15 May [O.S. 3 May] 1845 –15 July 1916) was a Russian zoologist best known for his pioneering research in immunology.

In particular, he is credited with the discovery of phagocytes (macrophages) in 1882. This discovery turned out to be the major defence mechanism in innate immunity. He and Paul Ehrlich were jointly awarded the 1908 Nobel Prize in Physiology or Medicine "in recognition of their work on immunity". He is also credited by some sources with coining the term gerontology in 1903, for the emerging study of aging and longevity. He established the concept of cell-mediated immunity, while Ehrlich that of humoral immunity. Their works are regarded as the foundation of the science of immunology. In immunology, he is given an epithet the "father of natural immunity".