A little more on immunity if anyone is interested. This stuff fascinates me almost as much as Supermodels.
A fishes epidermis consists of, besides other things, various types of unicellular glands some of which are goblet or mucous (slime) glands. These glands secrete slime. (other parts of fish also produce slime) Slime is a major part of the fishes immune system.
Besides secreting slime, the epidermis produces keratocytes. These motile keratocytes can cover the surface of a wound in hours after it's formation adding bacterial resistance to the wound and helping new cells to grow. This is the reason fish appendages, such as fins can re- grow.
Skin mucus evolved to trap and immobilize most bacteria and pathogens including parasites. This mucus is constantly shed, eliminating most pathogens and is immediately replaced.
The thickness of the slime is regulated in part by the stressers put on the animal such as temperature or stress. Copper, that we often use to treat fish would also cause the fish to produce more mucus there by helping the fish to eliminate the pest as well as to help kill it.
Interestingly a different type of mucus is produced for several days after a fish is stressed composed of a gel like material. It is unknown weather this gel has the same antibacterial and anti parasitic properties of the normal slime. This last statement has not been qualified on "all" types of fish as it has not been tested but it is interesting to me because I believe quarantining fish in a small tank would elicit this response making the fish more susceptical to infection. This is only a thought on my part and not something I discovered through research.
There are many more ways a fish has immunity to pathogens but they all have one thing in common. They require energy in the form of food. I know I have said it numerous times, but almost none of our fish are fed enough or correctly which is the reason for so many disease threads. With all the help fish get from their vast, complicated immune system, they should never get sick. If they do, it is our fault, not the store, not the wholesaler and not the fish.
We supply the living conditions, the stress and the food and most of us are not very good at it. Fish in our care should live out their normal lives, disease free and they should also be producing eggs, and if they are in the proper tank, spawning.
If our fish are not spawning, their immune system is not functioning and they are susceptible to all sorts of infections.
Feeding fish correctly is easy as I have mentioned many times. We have an obligation to keep these creatures in the state of health that they were living in, in the sea.
Just my opinion of course.
Reference:
ISRN ImmunologyVolume 2012 (2012), Article ID 853470, 29 pageshttp://dx.doi.org/10.5402/2012/853470
Review Article
An Overview of the Immunological Defenses in Fish Skin
María Ángeles Esteban