In the animal kingdom, innate immunity is the first line of defense against invading
pathogens. The dangers of microbial and parasitic attacks are countered by similar
mechanisms, involving the prototypes of the cell-mediated immune responses, the phagocytosis
and encapsulation. Work on Drosophila has played an important role in promoting an
understanding of the basic mechanisms of phylogenetically conserved modules of innate
immunity. The aim of this review is to survey the developments in the identification
and functional definition of immune cell types and the immunological compartments
of Drosophila melanogaster. We focus on the molecular and developmental aspects of
the blood cell types and compartments, as well as the dynamics of blood cell development
and the immune response. Further advances in the characterization of the innate immune
mechanisms in Drosophila will provide basic clues to the understanding of the importance
of the evolutionary conserved mechanisms of innate immune defenses in the animal kingdom.
