Short answers, pulled from the story.
Metchnikoff discovered phagocytosis in 1882, the process by which certain white blood cells surround and kill pathogens such as bacteria. The cells responsible he called phagocytes, specifically macrophages. This discovery became recognised as the major defence mechanism in innate immunity and the foundation of cell-mediated immunity.
Metchnikoff shared the 1908 Nobel Prize in Physiology or Medicine with Paul Ehrlich "in recognition of their work on immunity." Metchnikoff's contribution was the discovery of phagocytosis and the concept of cell-mediated immunity; Ehrlich established the concept of humoral immunity.
Metchnikoff developed a theory that aging was caused by toxic gut bacteria and that lactic acid bacteria could prolong life. He attributed the longevity of Bulgarian peasants to their yogurt consumption, specifically a bacterium now classified as Lactobacillus delbrueckii subsp. bulgaricus. He called this concept "orthobiosis," which became the basis of what is now known as probiotics.
Élie Metchnikoff, born Ilya Ilyich Mechnikov, was a zoologist from the Russian Empire of Moldavian noble ancestry. He was born in the village of Ivanovka, Kharkov Governorate, in a territory that is now Kupiansk Raion, Kharkiv Oblast in Ukraine. He later moved to France and spent the last decades of his life at the Pasteur Institute in Paris.
Metchnikoff coined the term gerontology in 1903 for the emerging scientific study of aging and longevity. He is sometimes called the "father of gerontology" for this contribution, though he acknowledged that such titles are often shared among multiple contributors in science.
Metchnikoff was not buried in a traditional grave. Following his will, his body was used for medical research after his death in Paris in 1916 and then cremated at Pere Lachaise Cemetery. His cinerary urn was placed in the library of the Pasteur Institute.