Invertebrate Immune Systems

Christopher J Bayne, Department of Zoology, Oregon State University, Corvallis, Oregon, USA Gerardo R Vasta, Center of Marine Biotechnology, Baltimore, Maryland, USA

Invertebrates, despite their lack of lymphoid immune systems, recognize and respond to nonself substances at least as efficiently as do vertebrates. Invertebrates rely on a diversity of mechanisms, some of which are inducible; however, the responses are in most cases short-lived and do not discriminate between individual pathogens. Therefore, responses mounted by invertebrates to potentially infectious agents are mediated by immune systems only in the sense that they resemble qualitatively the 'innate' or 'natural' immune responses of vertebrate myeloid cells and nonimmunoglobulin, humoral components. As invertebrates include about 95% of the extant animal spccies, and represent a vast diversity of organisms, from unicellular protozoans to the more complex echinoderms and protochordates, it is not surprising that a considerable range of strategies for recognition and defense against potential pathogens and parasites are to be found in these taxa.

Through his studies with starfish and water fleas in the nineteenth century, Metchnikoff established comparative immunology. His observation that foreign objects entering the body became covered with white blood cells (leukocytes) led him to consider phagocytosis and encapsulation as fundamental processes in immune defense. In the 1970s, Hildc-mann contributed substantively to the development of a global perspective on the evolution of cellular and humoral immune competence across the spectrum of animal diversity. More recent studies by Boman established the usefulness of invertebrate models for the understanding of antibacterial strategies, some of which have merited conservation throughout the evolution of the chordatc lineages.

Invertebrate internal defense responses exhibit common themes such as phagocytosis and encapsulation, but the underlying molecular recognition and effector mechanisms are diverse. Some of the factors involved, such as a2-macroglobulins (a2-M), C reactive proteins (CRP), antibacterial peptides, serine proteinases and proteinase inhibitors, have been substantially conserved through the evolutionary lineages leading to the chordates, whereas others, such as C-type lectins and complement-related factors, retained only those regions of the molcculc that are relevant to recognition/effector functions. Finally, for other factors such as glucan-binding proteins (crustaceans) and some antibacterial peptides, no homologs have been identified in vertebrates so far, and they appear to be restricted to invertebrates. The potential for finding novel molecules or mechanisms in invertebrates has focused attention on a growing number of species as model organisms, and progress has been substantial during recent years.

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