Teasing apart the mosquito immune system Hemocytes are key immune cells of insects, playing a major role in how vector species transmit disease. Raddi et al. collected and sequenced the RNA of more than 8000 individual hemocytes from the disease-carrying mosquitoes Anopheles gambiae and Aedes aegypti . These data were then analyzed to determine hemocyte differentiation lineages and population changes in response to noninfected and Plasmodium -infected blood meals. From these data, the researchers identified a new hemocyte type, the megacyte, defined by the transcription of specific marker genes. Gene-silencing experiments implied that the megacyte functions in hemocyte differentiation during immune priming and thus may be involved in the immune response in mosquitoes. Science , this issue p. 1128