[Dentoid]

Immunity is a highly complex cellular mechanism that is not isolated to internal tissues. Skin has a highly developed cellular immune response. There are immediate undifferentiated immune responses as well as "remembered" cellular immune responses mediated by immunoglobulins. For example, in humans, a classic example of immunoglobulin reaction is the Tuberculin Skin Test.

I'll admit that I know nothing of fish immunity, but with what I know about immunity and in my study of mammalian immunology I feel somewhat confident in extrapolating what is known about mammalian immunology to immunity in general. The question is not whether fish develop immunity to insults from skin parasites but more a question of how long does this immunity last. It's possible that in some fish the first infection instills immunity and they live through it, then the next infection kills them through an anaphylaxtic reaction. Much like insect bites in humans. Therefore, they developed an allergy. It is also possible that some species of fish develop immunoglobins that are secreted in the slime coat that preclude the parasite from getting a foothold.

I'm sure there is much more to it than this, but most deaths from the ich parasite is probably from asphixiation (sp) as the parasite infects the highly vascular gill tissue interfering with gas exchange.