Adaptive immune system factors

On the one hand, are the soluble factors such as lysozyme and complement, together with the phagocytic cells that contribute to the innate system, and on the other hand are the lymphocyte-based mechanisms that produce antibody and the T lymphocytes, which represent the main elements of the adaptive immune system. Not only do these lymphocytes provide improved resistance by repeated contact with a given infectious agent, but the memory with which they become endowed shows very considerable specificity to that infection. 

Abstract This chapter describes what is known about the main components and responses of the innate and adaptive immune system of fish. The chapter first reviews the organs, cells and molecules of the immune system known in a few economically important or model fish species. Molecular evidence suggests a similar immune system exists throughout the jawed vertebrates yet marked differences are also apparent. The innate parameters are at the forefront of fish immune defence and are a crucial factor in disease resistance. The adaptive response of fish is commonly delayed but is essential for long lasting immunity and a key factor in successful vaccination. 

Fish represent the earliest class of vertebrates in which both innate and acquired, or adaptive, immune mechanisms are present. The innate immune system appears to play a central role in the response to infections in fish, whereas in mammals the adaptive immune system is more significant. The intrinsic inefficiency of the adaptive immune response in fish is due to its evolutionary status - it only possesses IgM-like responses - and, moreover, due to environmental constraints such as temperature, because of the poi-kilothermic nature of fish. These factors result in a limited antibody repertoire, poor affinity maturation and memory, slow lymphocyte proliferation, and a short-lived secondary response. ... 

Flatworm parasites are well recognized for their ability to live for decades in environments where they are in contact with potentially damaging immune factors. This adaptability reflects the fact that these parasites have evolved mechanisms to evade immune effector mechanisms, and more remarkably, to sense and utilize components of the host immune system for their own development. Schistosomes, digenetic trematodes, are undoubtedly the most well-studied parasitic flatworms. These parasites infect over 200 million people in tropical and subtropical zones, and cause severe disease in approximately 5% of those infected. Of immunological interest is the fact that pathology in schistosome-infected individuals is caused largely by the immune response... 

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