The integumental defenses provide a physical and chemical barrier to the attachment and penetration of microbes. Besides the entrapping and sloughing of microbes in the mucus, the latter contains many antibacterial substances including anti-bacterial peptides, lysozyme, lectins and proteases. The gastro-intestinal tract is a hostile environment of acids, bile salts and enzymes able to inactivate and digest many viruses and bacteria. In most cases the integumental defenses are sufficient to protect against even quite virulent organisms which often only produce disease when the integument has been physically damaged. If a microbe gains access to the tissues of the fish, it is met with an array of soluble and cellular defenses. The complement system, present in the blood plasma, plays a central role in recognising bacteria and its activated products may lyse the bacterial cells, initiate inflammation, induce the influx of phagocytes and enhance their phagocytic activity. Complement can be activated directly by bacterial products and constituents and also indirectly by other factors, principally C-reactive protein and lectins, which can also bind to the bacterial surface. Plasma also contains a number of factors which inhibit bacterial growth(e.g. transferrin and anti-proteases) or which are bactericidal e.g. lysozyme. Following the infection of fish with virus pathogens, infected cells produce interferon. This induces antiviral defenses in neighbouring cells which are then protected from becoming infected. Anti-viral cytotoxic cells are able to lyse virally infected cells and thus reduce the rate of multiplication of virus within them. Innate defenses thus provide a pre-existing and fast-acting system of protection which is non-specific and relatively temperature-independent and thus has several advantages over the slow-acting and temperature-dependent specific immune responses.