Abstract
The immune system is composed of two subsystems—the innate immune system and the adaptive immune system. The innate immune system is the first to respond to pathogens and does not retain memory of previous responses. Innate immune responses are evolutionarily older than adaptive responses and elements of innate immunity can be found in all multicellular organisms. If a pathogen persists, the adaptive immune system will engage the pathogen with specificity and memory. Several components of the adaptive system including immunoglobulins (Igs), T cell receptors (TCR), and major histocompatibility complex (MHC), are assumed to have arisen in the first jawed vertebrates—the Gnathostomata. This review will discuss and compare components of both the innate and adaptive immune systems in Gnathostomes, particularly in Chondrichthyes (cartilaginous fish) and in Osteichthyes [bony fish: the Actinopterygii (ray-finned fish) and the Sarcopterygii (lobe-finned fish)]. While many elements of both the innate and adaptive immune systems are conserved within these species and with higher level vertebrates, some elements have marked differences. Components of the innate immune system covered here include physical barriers, such as the skin and gastrointestinal tract, cellular components, such as pattern recognition receptors and immune cells including macrophages and neutrophils, and humoral components, such as the complement system. Components of the adaptive system covered include the fundamental cells and molecules of adaptive immunity: B lymphocytes (B cells), T lymphocytes (T cells), immunoglobulins (Igs), and major histocompatibility complex (MHC). Comparative studies in fish such as those discussed here are essential for developing a comprehensive understanding of the evolution of the immune system.
Highlights
The vertebrate immune system is divided into 2 subsystems—the innate immune system and the adaptive immune system
Innate immunity is present in all multicellular organisms, an adaptive immune system, based on variable lymphocyte receptors (VLRs), B-like, and T-like cells is found in jawless vertebrate, while an adaptive immune system, based on an Ig/T cell receptors (TCR)/major histocompatibility complex (MHC) system, evolved with the appearance of jawed vertebrates
There is limited knowledge, compared to mammals, on the homing and recirculation of B and T cells in ray-finned fish, cartilaginous fish, and lobe-finned fish. Much of this knowledge will only be gained with the development of the appropriate reagents and techniques
Summary
The vertebrate immune system is divided into 2 subsystems—the innate immune system and the adaptive immune system. A recent study by Heimroth et al [20] identified an increase in proteins with known antimicrobial function including histones and S100 proteins in skin mucus of the lungfish P. dollo during terrestrialization In both fish and mammals, tissue injury, infection and inflammation induce immune cells, such as macrophages, to secrete various cytokines into the bloodstream, which stimulate hepatocytes to produce and release acute phase proteins (APPs) [141, 142]. IgM is the most prevalent Ab in both bony and cartilaginous fish plasma and can be found in both secreted and transmembrane forms It shares a similar function in all jawed vertebrates, which includes mediating opsonization, antibody-dependent cell-mediated cytotoxicity, and complement activation, and contributes to both innate and adaptive immune responses [58, 102, 174, 175]. Both IgN and IgQ are thought to originate from the IgW lineage [160]
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