Abstract
Lower vertebrates like fish mostly depend on innate immunity as the first line of defence against invading pathogens. One of the defence mechanisms in innate immunity for fish is the use of phagocytic cells such as monocytes, macrophages, neutrophils, mast cells and dendritic cells in fighting pathogens. These cellular cells employ a variety of mechanisms in protecting fish including but not limited to; the use of pattern recognition receptors and antigen presenting cells to detect the presence of pathogens in host, neutralising them using measures including; inflammation, production of lysosomal enzymes released into the phagosome to neutralising pathogens and prevention of systemic autoimmunity by apoptosis. Some phagocytic cells play the extra role of serving as a bridge between innate and adaptive immunity in fish. Recent accumulating data showed that phagocyte cell activity in fish could be enhanced by a natural or chemical substance (immunostimulants) that stimulates the immune system in fish principally meant to enhance the immune response mechanisms. In this review, the main focus is on elucidating the distinct phagocytic mechanisms played by monocytes, macrophages, neutrophils, mast cells and dendritic cells as principal phagocytes in immune defence and how their activities are influenced by immunostimulants in cultured fish.