The innate immune system of fish is considered to be the first line of defence against a broad spectrum of pathogens and is more important for fish as compared with mammals. Lysozyme level or activity is an important index of innate immunity of fish and is ubiquitous in its distribution among living organisms. It is well documented that fish lysozyme possess lytic activity against both Gram-positive bacteria and Gram-negative bacteria. It is also known to be opsonic in nature and activates the complement system and phagocytes. It is present in mucus, lymphoid tissue, plasma and other body fluids of freshwater and marine fish. It is also expressed in a wide variety of tissues. Lysozyme activity has been shown to vary depending on the sex, age and size, season, water temperature, pH, toxicants, infections and degree of stressors. Here, we review our current understanding of different types of lysozyme and their expression and its role in fish innate immune system.

Lysozyme: an important defence molecule of fish innate immune system

Current research status of fish immunostimulants

Some medicinal plants as immunostimulant for fish

Tissue trauma or invasion by pathogens or parasites induce changes in the quantities of several macromolecules in animal body fluids These changes comprise one aspect of the acute phase response (APR), which in toto involves metabolic changes in several organ systems One clear indication of the response is the increase in synthesis and secretion by the liver of several plasma proteins, with simultaneous decreases in others These acute phase proteins (APP) function in a variety of defense-related activities such as limiting the dispersal of infectious agents, repair of tissue damage, inactivation of proteases, killing of microbes and other potential pathogens, and restoration of the healthy state Some APP are directly harmful to microbes, while others modify targets thus marking them for cell responses Some work alone while others contribute to cascades Proteins that are APP in mammals, and that have been identified in both teleosts and elasmobranchs include C-reactive protein, serum amyloid P, and several components of the Complement system Others reported in teleosts include transferrin and thrombin Of these, only CRP has been reported to increase in acute phase plasma In trout, a precerebellin-like protein is an APP with unknown functions A cDNA library enriched in fragments of transcripts that were more abundant in livers from fish undergoing an APR recently yielded sequences resembling 12 additional known APP, and as many others either not known to be APP, or not similar to others yet in public databases It appears that, as in mammals, hepatocytes are the prime source of APP in fish, and that pro-inflammatory cytokines induce transcription of their genes

Erling Frivold

Papers

Yeast glucan induces increase in lysozyme and complement-mediated haemolytic activity in Atlantic salmon blood