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Both non-specific and specific factors in the humoral and cellular branches of the fish immune system are well developed. Thus, production of specific antibodies in teleost B-lymphocytes against natural or synthetic antigens has been well characterized. Recent studies have also described T-cell markers and T-cell activity in a limited number of fish species. MHC class I and II genes from a number of fishes have been sequenced indicating a well developed adaptive immune system in fish. The activity of leukocytes is not restricted to phagocytosis but does also include respiratory burst activity (production of reactive oxygen and nitrogen species) and the release of reactive mediators upon stimulation with antigens. Non-specific cytotoxic cells together with macrophages, mast cell equivalents (eosinophilic granular cells) and neutrophils seem to play a central role in fish immunology. Important humoral factors such as complement, C-reactive protein, lectins and cytokines are also significant players in the intricate immune system in fish. Further, these protective mechanisms have been studied in fish exposed to various parasites and it is clear that a range of effector arms are responsible for the protection of teleosts against parasitic invasion. Immune evasion mechanisms are known in parasite host systems and are likely to interfere to some degree with future anti-parasitic vaccines. Despite this studies including protists such as Ichthyophthirius multifiliis and Trypanoplasma and metazoans such as monogeneans and digeneans indicate that teleosts can be partially protected against parasite invasion if the hosts are vaccinated at a strategic correct time.