The paper is devoted to the biography of a Russian émigré scientist Sergei Ivanovich Metalnikov. The new details of the scientist’s life and professional activities are reviewed, based on the archive and published documents. A number of documents from Metalnikov’s epistolary and journalistic heritage were introduced for scientific use. Some earlier unknown details of his biography were reconstructed, related to his scientific, science-organising, teaching, and public activities. The scientist’s journalistic and philosophical works are reviewed and it is shown how Metalnikov’s scientific and philosophical views formed the basis for his socio-political views. Metalnikov’s scientific biography is reconstructed in great detail. Based on primary sources, the main areas of his scientific work in the different periods of his life, as well as the aspects of formation of his scientific interests are identified and reviewed in detail, with the biggest emphasis placed on his numerous scientific achievements in immunology and his contribution to the making and development of this science. The results of Metalnikov’s studies devoted to serum toxicity, insect pathology and immunology, biological methods of pest control, and the link between nervous system and immunity are reviewed in detail. Attention is also paid to the results in related areas and it is shown how these are connected with the studies on immunity; the place of the studies on phagocytic mechanisms in Metalnikov’s works is elucidated. The Appendix contains Metalnikov’s scientific autobiography.