摘要：

This chapter surveys the classification of hemocytes and their origins and functions. Insect hemocytes are identified by a combination of morphological, antigenic, and functional characteristics. Although hemocytes have similar functions in immunity across all insects, the naming of different hemocyte types varies somewhat among species and taxa. Drosophila larvae contain three terminally differentiated types of hemocytes named plasmatocytes, crystal cells, and lamellocytes. In most other insects, the main differentiated hemocytes described in circulation are named granulocytes. Granulocytes are morphologically distinguished by the granules in their cytoplasm, their ability to strongly adhere and spread on foreign surfaces in primary culture, and their tendency to spread symmetrically. Hemocytes have been described from species in many other taxa, including Orthoptera, Blattaria, Coleoptera, Hymenoptera, Hemiptera, and Collembola. Key challenges in functionally studying hemocytes in many species include the small size of many insects that often makes sample collection difficult. The first population of hemocytes is produced during embryogenesis from head or dorsal mesoderm, while the second is produced during the larval or nymphal stages in mesodermally derived hematopoietic organs. Studies with a number of insects indicate that the number of hemocytes in circulation can also rapidly increase in response to stress, wounding, or infection. The main immune defenses mediated by hemocytes are phagocytosis, encapsulation, and clotting. The mammalian immune system recognizes viral pathogens at the cell surface or in endosomal vesicles through pattern recognition receptors such as Toll-like receptors. Recognition of viruses and intracellular antiviral defense mechanisms could obviously involve a variety of cell types in addition to hemocytes. However, hemocytes are likely important in antiviral defense given that most insect viruses disseminate through the hemocoel during the course of a systemic infection.

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