Fish immune system. A crossroads between innate and adaptive responses

Fish, as the first vertebrate group appearing in evolution after adaptive radiation during the Devonic period, still represent the most successful and diverse group of vertebrates. This heterogeneous group of organisms occupy an apparent crossroads between the innate immune response and the appearance of the adaptive immune response. Importantly, immune organs homologues to those of the mammalian immune system are present in fish. However, their structural complexity is less, potentially limiting the capability to generate fully functional adaptive immune responses against pathogen invasion. The ability of fish to mount successful immune responses with apparently more robust innate responses than that observed in higher vertebrates is discussed.