In his fundamental work 'Immunity in Infectious Diseases' (1901), Mechnikov systematized his years of research and presented a comprehensive phagocytic theory of immunity. He proved that phagocytes — macrophages and microphages (neutrophils) — are key elements of the body's defense against infections. This theory complemented Paul Ehrlich's humoral theory, and both approaches together formed the foundation of modern immunology. For this work, Mechnikov and Ehrlich jointly received the 1908 Nobel Prize.
Significance
Fundamental understanding of the human immune system