Immunology of Invertebrates: Cellular

Valerie J Smith, University of St Andrews, Fife, Scotland

Published online: April 2010

DOI: 10.1002/9780470015902.a0002344.pub2

Circulating (free) cells are the main mediators of immunity in invertebrates, carrying out the phagocytic, encapsulating and microbicidal responses that enable these animals to protect themselves against infection without a specific adaptive immune system. The cells are specialised for these functions, with division of labour between types. In general, immune cell diversity tends to increase with body complexity and life histories but common to the majority of species, however, are phagocytic and granular cell types that synthesise and store bioactive proteins. The release of these compounds occurs by exocytosis following interaction of pattern recognition receptors and nonself motifs carried on the surface of pathogens or parasites. Cell death of one form or another appears to be inherent in immune reactivity of many invertebrates with new cells produced, at least in coelomates, by mesodermal haemopoietic tissues.

Key concepts

  • Immunity in invertebrates is confined to nonspecific inflammatory responses, mediated to a large extent by the circulating blood cells (haemocytes or coelomocytes) or their products.
  • All coelomate invertebrates contain populations of freely circulating cells dedicated to host defence, some well developed for specialist purposes.
  • There are no true vertebrate-type lymphocytes and no long-term, highly specific immune memory in invertebrates.
  • The main invertebrate cellular immune responses include phagocytosis, encapsulation, cytotoxicity and the synthesis and release of microbicidal agents.
  • These cellular reactions often involve the death of immune cells or their removal from the circulation, requiring new cells to be synthesised in the haemopoietic tissue and released into the blood.
  • Genomic and proteomic studies have enabled many of the proteins responsible for cellular defence in invertebrates to be identified, especially in arthropods.
  • Although many of the responses and pathways are ancient and highly conserved, there is great diversity in the effector molecules across the invertebrates as a whole.

Keywords: invertebrate immunity; blood cell; haemocytes and coelomocytes; prohaemocyte; phagocytes; encapsulation reactions; evolution of immunity; primordial immunity; comparative immunology; cellular defences

Figure 1. Highly stylised graphic representation of the main blood cell types involved in the immune responses of invertebrates: (a) Prohaemocyte: an immature cell present in the circulation of decapod crustaceans and insects, and probably within the haemopoietic tissue of other invertebrates. Cells with similar appearance, called lymphocyte-like cells, also occur in ascidians. (b) Amoeboid phagocyte: cells of this type are present in anthozoans, annelids, insects (where they are called plasmatocytes), echinoderms and ascidians. (c) Granular cell: Mature cells that synthesise and exoctose many bioactive factors. They are present in decapod crustaceans, insects, bivalves and chelicerates. (d) Hyaline cell: a haemocyte often phagocytic in decapod crustaceans and bivalve molluscs. (e) Semigranular cell: a highly labile haemocyte present in decapod crustaceans. (f) Spherule or morula-type cell: haemocytes or coelomocytes that synthesise, transport and release various factors in some cnidarians, annelids, insects, echinoderms and ascidians. (g) Cystocyte (sometimes called thrombocytoid): ‘explosive corpuscle’ present in insects that are involved in clotting and melanisation. (h) Oenocytoid: a storage cell that is involved in melanisation in some insects. Modified oenocytoids, called crystal cells, occur in dipterans. (i) Vibratile cell: a motile cell unique to echinoid and some holothurian echinoderms that contributes to clotting. (j) Signet ring cell: a vacuolated coelomocyte present in ascidians. (k) Compartment cell: a multivacuolated coelomocyte present in ascidians. Not to scale. Drawings by EA Dyrynda.
Figure 2. Light micrographs of crab, Carcinus maenas haemocytes. (a) Hyaline cell stained with Wrights stain. (b) Granular and (c) Semi-granular cells stained with Wrights stain. (d) Haemocyte showing condensation of the nucleus, blebbing and fragmentation typical of a cell undergoing apoptosis (Wrights stain). (e) Aradite section of a haemocyte capsule formed in a gill filament 12 h after injection of Gram-positive bacteria into the haemocoel. The capsule has layers of flattened haemocytes around a central, necrotising core. Both granular and hyaline cells can be seen still attaching to the outside of the structure. Bacterial cells can be seen enclosed within the cell matrix (arrows). (a)–(d) Courtesy of C Robb and EA Dyrynda. (e) Reproduced from Smith and Ratcliffe (1980), with permission from Elsevier.
Figure 3. Highly stylised schematic representation of the cellular immune activities of invertebrates following the entry of microorganisms into the body. This scheme is based loosely around a typical decapod model. Hollow arrows indicate cell behaviours or changes; dashed arrows represent possible cell death events; dotted arrow and box indicate a magnified detail of Lysosomal fusion to the phagosome. Not to scale.
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Innate Immunity | Cells of the Immune System
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Article Title: Immunology of Invertebrates: Cellular
 
How to Cite close
Smith, Valerie J(Apr 2010) Immunology of Invertebrates: Cellular. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002344.pub2]