Russian immunogist Elie Metchnikoff discovered that many microorganisms could be engulfed and digested by phagocytic cells, which he called macrophages. These macrophages fight a large range of pathogen without requiring prior exposure. Thus, macrophages are considered innate because they all act in the same way in normal fish. Environmental temperature influences all aspects of fish physiology, including the immune response. Thus, fish are called poikilothermic.
Macrophages develop from bone marrow precursors which mature and enter the bloodstream as monocytes.
Macrophages secrete signalling molecules called cytokines and chemokines which orchestrate the immune response. Cytokines are small proteins which allow cells of the immune system to communicate with one another via cytokine receptors expressed at the cell surface. Some cells can take up small particulate matter by surrounding the particle with extrusions of the cell membrane by a process termed phagocytosis. Macrophages have great phagocytosis ability. Macrophages in culture will phagocytose a whole range of different particles from small latex beads to polymer beads twice their own diameter.