Mosquito hemocyte-mediated immune responses

•A major component of the mosquito immune system is hemocytes.•Hemocytes are anatomically divided into circulating and sessile hemocytes.•Hemocytes are functionally divided into granulocytes, oenocytoids and prohemocytes.•Hemocytes kill pathogens via phagocytosis, melanization and lysis. Hemocytes are a key component of the mosquito immune system that kill pathogens via phagocytic, lytic and melanization pathways. Individual mosquitoes contain between 500 and 4000 hemocytes, which are divided into three populations named granulocytes, oenocytoids and prohemocytes. Hemocytes can also be divided by their anatomical location with 75% of hemocytes circulating in the hemocoel (circulating hemocytes) and 25% of hemocytes attaching themselves to tissues (sessile hemocytes). Greater than 85% of the hemocytes in adult mosquitoes are granulocytes, which primarily kill pathogens by phagocytosis or lysis. Oenocytoids, on the other hand, are the major producers of the enzymes required for melanization while prohemocytes are small cells that participate in phagocytosis. Both circulating and sessile hemocytes engage in defense against pathogens. The circulatory system of mosquitoes also interacts with hemocytes and facilitates elimination of potential pathogens that enter the hemocoel.