Antiparasitic immune responses employ a common set of host immune factors but display a large variation over the theme. The number of fish species counts far more than 33,000, and due to the high host specificity of parasites, the number of fish parasites is probably even higher. In addition, the parasite group comprises protozoans (amoebae, flagellates, ciliates) and metazoans (myxozoans, monogeneans, cestodes, trematodes, nematodes, acanthocephalans, crustaceans) representing a diversity of life forms. These parasite types apply a broad field of mechanisms to modulate the host response optimizing survival of the parasite species in question. Despite these differences among parasite groups, a common set of immune reactions are activated upon infection although variations occur due to the highly varying surface molecules and excretory/secretory compounds released during the infection process. These are recognition of PAMPS by PRR, subsequent signal transduction and initiation of inflammatory responses, the expression of innate protective effector molecules, antigen presentation and establishing adaptive T- and B-cell responses. The host immune reactions can control the infection, but immune-modulating mechanisms have evolved to secure an optimal survival of the parasite type securing survival and spread of the species. The local release of immune-modulating substances in the microhabitat may explain why established parasites survive, while younger invading parasite stages fail to reach their destination. This opens the perspectives for the production of antiparasitic vaccines in the future.