Fish Health Management Division, Central Institute of Freshwater Aquaculture, Kausalyaganga, Bhubaneswar-751002, India.
The non-specific or innate immune system is the only weapon in invertebrates that renders protection against infections, whereas it is a fundamental defence mechanism in vertebrates playing instructive role in the acquired immune response. Innate defence is fast-acting and pre-existing system of protection which is relatively temperature-independent. The non-specific immune system consists of integumental defence providing physical and chemical barrier to attachment and penetration of microbes, and humoral and cellular components. Besides the entrapment and sloughing of microbes in the mucus, the latter contains many antimicrobial substances including antibacterial peptides, lysozyme, lectins, complement, proteases and immunoglobulins. Non-specific humoral factors include growth inhibiting substances, e.g. transferrin and antiproteases; lysins, e.g. lysozyme, C-reactive protein (CRP), antibacterial peptides and complement which has lytic, chemotactic, opsonic and proinflammatory properties; and agglutinins, precipitins, natural antibodies, cytokines and chemokines. The cells play major role in innate defence are phagocytic cells (neutrophils and macrophages) which digest or degrade pathogens by production of reactive oxygen species (ROS), reactive nitrogen species (RNS) as well as by utilizing their enzymatic system (lysozyme and myeloperoxidase). The recognition of pathogens by innate system is being served by a limited number of germ-line encoded pattern recognition proteins or receptors which recognize pathogen associated molecular patterns (PAMPs) like bacterial and fungal glycoproteins, lipopolysaccharides and intracellular components released by damaged cells. The non-specific defences against viruses may be broadly categorized into constitutive comprising action of non-specific cytotoxic cells to virus infected cells, membranolytic activity of complement lysing virus particles, genetic make up of host, tissue-tropism, and responsive mechanism by inducing interferon production and subsequent enhanced transcription of interferon-stimulated genes (viz., PKR, TLR3, RIG-1, Mda-5, Mx) important for antiviral defence.
