In his early days, he cooperated with Russian zoologist α ο Kovalevski to study the development of embryonic germ layer of invertebrates. 1880, his wife's hometown farm was infested with beetles, and he experimented with fungi to control beetles, which made him interested in studying infectious diseases. In 1862, German biologist ernst haeckel described the phenomenon that white blood cells engulf dye particles. In 1865, Mechnikov noticed the intracellular digestion of ascaris lumbricoides. In 1882, he observed that there was a transparent and movable cell in the starfish larvae, which surrounded the invading foreign bodies, such as indigo dye particles. This kind of cell comes from mesoderm, which is similar to the inflammatory reaction of animals with vascular system. Therefore, he believes that this cell has a defensive function. This discovery was published in 1883, and the term "phagocyte" was coined. Later, he observed that the wandering cells of starfish larvae can absorb useless body parts during the metamorphosis of larvae, which proved that the white blood cells of higher animals (including people) also come from mesoderm and can remove invading foreign bodies, especially bacteria. Many people oppose this theory, thinking that phagocytes do not protect the body, but bring invading foreign bodies around the body. From 1883 to 19 10, he wrote many articles to defend and revise his theory, such as Comparative Pathology of Inflammation (1892) and Immunity of Infectious Diseases (190/kloc-0). After entering the Pasteur Institute, he continued to study the mechanisms of immunity, fever and infection, and gave public lectures, which attracted many students, including J. Bordais. Later, A.E. Wright and S.R. Douglas discovered penicillin, which can help white blood cells phagocytize bacteria. This discovery also contributed to the establishment of cellular immunity theory.

1903 He and E. Lu infected apes with syphilis, which proved that human non-treponema pallidum was the only host. Through the study of aging and longevity, it is considered that good hygiene habits and normal intestinal flora are conducive to health and longevity. The longevity of Bulgarians is also related to their habit of eating yogurt containing lactic acid bacteria. Human nature published in 1903.