Élie Metchnikoff, Ukrainian-French biologist and zoologist, Nobel Prize laureate (d. 1916)
Ilya Ilyich Mechnikov (Russian: Илья Ильич Мечников, also written as Elie Metchnikoff; 15 May [O.S. 3 May] 1845 – 15 July 1916) was a Russian zoologist of Romanian nobility ancestry and Ukrainian Jewish origin best known for his pioneering research in immunology. He and Paul Ehrlich were jointly awarded the 1908 Nobel Prize in Physiology or Medicine "in recognition of their work on immunity". He was born, lived and worked for many years on the territory of the Russian Empire. Given this complex heritage, four different nations and peoples justifiably lay claim to Mechnikov.Honoured as the "father of innate immunity," Mechnikov was the first to discover a process of immunity called phagocytosis and the cell responsible for it, called phagocyte, specifically macrophage, in 1882. This discovery turned out to be the major defence mechanism in innate immunity, as well as the foundation of the concept of cell-mediated immunity, while Ehrlich established the concept of humoral immunity to complete the principles of immune system. Their works are regarded as the foundation of the science of immunology.Mechnikov developed one of the earliest concepts in ageing, and advocated the use of lactic acid bacteria (Lactobacillus) for healthy and long life. This became the concept of probiotics in medicine. Mechnikov is also credited with coining the term gerontology in 1903, for the emerging study of aging and longevity. In this regard, Ilya Mechnikov is called the "father of gerontology" (although, as often happens in science, the situation is ambiguous, and the same title is sometimes applied to some other people who contributed to aging research later).

Supporters of life extension celebrate 15 May as Metchnikoff Day, and used it as a memorable date for organizing activities.