The primary hemocytes involved in encapsulation are plasmatocytes and lamello-cytes. Plasmatocytes comprise much of the early layers of immune cells surrounding parasite eggs, and phagocytose proteins from the wasp long gland that are deposited in the Drosophila hemocoel with the parasitic egg. Lamellocytes are present in the earlier cell layers, and these are the cells that flatten and form the later layers during encapsulation (Russo et al. 1996). Lamellocytes usually make up less than 5% of circulating hemocytes (Rizki and Rizki 1992), but increase in number following wounding and parasitism. This proliferation results from mitosis of lamellocyte precursors in the hematopoietic organ (Sorrentino et al. 2002). Recent studies have shown that lamellocyte proliferation is targeted by parasites. Virulent wasps parasi-toids induced atrophy of hematopoietic organs (Chiu et al. 2001; Moreau et al. 2003), indicating that the proliferation response is likely necessary for a successful immune response. Further evidence of the necessity of lamellocyte proliferation was found in a comparison of virulent and avirulent parasites in Drosophila. Lamellocyte proliferation was greater after egg deposition by avirulent parasites than after sterile wounding or egg deposition by virulent parasites. Interestingly, the lamellocyte proliferation following sterile wounding is greater than that following egg deposition by virulent parasites (Russo et al. 1996). This result suggests that virulent parasites deposit their eggs with factors that block proliferation.
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