[13] Mechnikov was born in a region of the Russian Empire that is today part of modern-day Ukraine to a Moldavian noble father[4] and a Ukrainian-Jewish mother,[14] and later on continued his career in France.

[19] Metchnikoff developed one of the earliest concepts in ageing, and advocated the use of lactic acid bacteria (Lactobacillus) for healthy and long life.

[21][22] In this regard, Ilya Mechnikov is called the "father of gerontology"[23][24] (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).

[25][26] Metchnikoff was born in the village of Ivanovka [uk], Kharkov Governorate, in the Russian Empire, now located in Kupiansk Raion, Kharkiv Oblast in Ukraine.

[12] The family name Mechnikov is a translation from Romanian, since his father was a descendant of the Chancellor Yuri Stefanovi, the grandson of Nicolae Milescu Spătarul.

It was in Leuckart's laboratory that he made his first scientific discovery of alternation of generations (sexual and asexual) in nematodes (Chaetosomatida) and then at the University of Munich.

Moving to Naples the next year he worked on a doctoral thesis on the embryonic development of the cuttle-fish Sepiola and the crustacean Nebalia.

A cholera epidemic in the autumn of 1865 made him move to the University of Göttingen, where he worked briefly with W. M. Keferstein and Jakob Henle.

Together they won the Karl Ernst von Baer prize for their theses on the development of germ layers in invertebrate embryos.

After being involved in a conflict with a senior colleague over attending scientific meetings, he transferred to the University of Saint Petersburg in 1868, where he experienced a worse professional environment.

He discussed his hypothesis with Carl Friedrich Wilhelm Claus, Professor of Zoology at the University of Vienna, who suggested to him the term "phagocyte" for a cell which can surround and kill pathogens.

[12] His theory, that certain white blood cells could engulf and destroy harmful bodies such as bacteria, met with scepticism from leading specialists including Louis Pasteur, Emil von Behring, and others.

[13] He worked with Émile Roux on calomel (mercurous chloride) in ointment form in an attempt to prevent people from contracting the sexually transmitted disease syphilis.

The so-called resolution of the humoralist and cellularist positions by showing their respective roles in the setting of enhanced killing in the presence of opsonins was popularized by Wright after 1903, although Metchnikoff acknowledged the stimulatory capacity of immunosensitized serum on phagotic function in the case of acquired immunity.

Some 85 years after this seminal observation, laboratory studies showed that these elements were low molecular weight (between 150 and 1500 Dalton (unit)s) N-formylated oligopeptides, including the most prominent member of this group, N-Formylmethionine-leucyl-phenylalanine, that are made by a variety of replicating gram positive bacteria and gram negative bacteria.

[34][35][36][37] Metchnikoff's early observation, then, was the foundation for studies that defined a critical mechanism by which bacteria attract leukocytes to initiate and direct the innate immune response of acute inflammation to sites of host invasion by pathogens.

During the 1892 cholera epidemic in France, he was surprised by the fact that the disease affected only some people but not others when they were equally exposed to the infection.

[41] Metchnikoff developed a theory that aging is caused by toxic bacteria in the gut and that lactic acid could prolong life.

[20][45] Metchnikoff won the Karl Ernst von Baer prize in 1867 with Alexander Kovalevsky based on their doctoral research.