Abstract Tambaqui is the second native fish most produced species in Brazil. Currently, tambaqui fish farms deals with serious sanitary problems due to the prevalence of the parasite Neoechinorhynchus buttnerae. However, the prevalence of the acanthocephalan parasite infections depends on the resistance and tolerance interactions between the host organisms and parasites. The immune response against parasites is divided between innate and acquired immunity. The innate defense is a result of physical barriers, cellular and humoral compounds. Acquired defense occurs through the production of antibodies (humoral) and is mediated by cells, mainly by type 2 T helper lymphocytes. Most parasites secrete a variety of immunomodulatory compounds that allow coexistence with the host and chronicity of the parasite. The host-parasite relationship is complex and makes prevention and treatment difficult. However, some studies show that the use of immunostimulants may have “systemic” effects. These include improvement of the intestinal mucosa health and also in the production of cellular and humoral compounds in the whole body, thus assisting treatment and control. As such, it is important to understand the mechanisms of resistance and tolerance in the host organisms so that prevention and treatment measures can be effective.

Abstract Molecular oxygen is a necessary compound for all aerobic organisms, although oxygen is a potent oxidant, which can cause oxidative stress (OS). OS occurs when there is an imbalance between the production of oxidant and antioxidants components, are result of normal cell metabolism, and many of these compounds play a fundamental role in several metabolic pathways. The organism produces several reactive oxygen species (ROS), but they are balanced by an antioxidant defense system that maintains the levels of these oxidizing compounds at an acceptable level. Many of these components are essential in the organism defense and their byproducts are considered potent bactericides that actively act in the destruction of invading pathogens. Fish immune system is composed of innate and acquired mechanisms of defense. Phagocytosis is an innate process of defense, which interconnects these two systems, since the pathogens processing by professional phagocytes is a fundamental stage for antibodies production. During phagocytosis there is production of ROS and consequent production of free radicals (FR), these compounds lead to the formation of potent bactericides to combat microorganisms. However, it is known that OS limits the immune response, with an impairment in defense compounds in an attempt to decrease the ROS production. Studies of fish FR production are preliminary and should be executed to evaluate the effects of ROS on fish, including their beneficial action against pathogens and its deleterious action on the oxidation of cellular components.