Cellular immune response of Drosophila melanogaster against Asobara tabida

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Abstract
The successful cellular immune response of larvae of D. melanogaster against A. tabida is characterized by an increase in the number of blood cells, a premature transformation of spherical plasmatocytes to flattened, disc-shaped lamellocytes, and the lysis of crystal cells. The plasmatocytes and transformed lamellocytes rapidly aggregate around the eggs of the parasite to form a cellular capsule, while the lysing crystal cells release substances causing the melanization of the capsule. The early appearance of large numbers of transformed blood cells in the haemolymph prior to their involvement in capsule formation suggests that transformation is independent of cell contact with the parasite and may be due instead to biochemical stimuli emitted from the parasite and/or by hormonal changes. Quantitative blood cell studies of hosts exhibiting no encapsulation and melanization response indicate that successfully developing parasites are recognized by the immune system but are able sufficiently to block the blood cell transformation and thus prevent capsule formation.