RESUMEN Considerados como la base evolutiva vertebrada tras su radia- ción adaptativa en el Devónico, los peces constituyen en la actua- lidad el grupo más exitoso y diversificado de vertebrados. Como grupo, este conjunto heterogéneo de organismos representa una aparente encrucijada entre la respuesta inmunitaria innata y la aparición de una respuesta inmunitaria adaptativa. La mayoría de órganos inmunitarios de los mamíferos tienen sus homólogos en los peces. Sin embargo, su eventual menor complejidad estruc- tural podría potencialmente limitar la capacidad para generar una respuesta inmunitaria completamente funcional frente a la inva- sión de patógenos. Se discute aquí la capacidad de los peces para generar respuestas inmunitarias exitosas, teniendo en cuenta la robustez aparente de la respuesta innata de los peces, en com- paración con la observada en vertebrados superiores. PALABRAS CLAVE: Inmunidad innata / Inmunidad adaptati- va / Vertebrados / Peces / Evolución. ABSTRACT Fish, as the first vertebrate group appearing in evolution after adap- tive radiation during the Devonic period, still represent the most suc- cessful 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. Impor- tantly, immune organs homologues to those of the mammalian immu- ne system are present in fish. However, their structural complexity is less, potentially limiting the capability to generate fully functional adap- tive immune responses against pathogen invasion. The ability of fish to mount successful immune responses with apparently more robust inna- te responses than that observed in higher vertebrates is discussed. KEY WORDS: Innate immunity / Adaptive immunity / Vertebrates / Fish / Evolution. 277 Revisión Inmunología Vol. 22 / Núm 3/ Julio-Septiembre 2003:277-286 Fish immune system. A crossroads between innate and adaptive responses L. Tort, J.C. Balasch, S. Mackenzie Department of Cell Biology, Physiology and Immunology, Universitat Autónoma de Barcelona, Bellaterra, Spain EL SISTEMA INMUNITARIO DE LOS PECES. UNA ENCRUCIJADA ENTRE LAS RESPUESTAS INMUNITARIAS INNATAS Y ADAPTATIVAS INTRODUCTION A recent estimation assumes that about 10 29 prokaryotic cells exist in oceans (1) , which are mainly responsible for the marine biomass. The aquatic media not only allows transport but also the growth of microorganisms. Thus, a number of trophic chains are less productive due to the ubiquity of heterotrophic bacteria which utilize 70% of the marine biogenic carbon (2) and viruses that may account for 10 10 cells/L in aquatic habitats (3) . The adaptation of autocatalytic functions is a coevolutionary process, which in case of the immune system involves competition related to the number of microorganisms and their pathogenicity. Although most effector mechanisms of allorecognition in invertebrates are not known, recent studies in comparative immunology stress two general patterns: (a) conservation of the defensive signal pathways that are relevant to nonspecific immunity, and (b) restriction of adaptive immunity to vertebrates (Fig. 1).