The defence against infections is the essential function of the immune system of all Vertebrates. Fish present an efficient and developed immunologic response and in the case of Teleosts, with some similarities to the higher vertebrates. It can be divided into two types: innate or non specific and adquired or specific, both with a humoral and cellular compartments. Their relative importance can change according to the age of fish and is always under the influence of different factors. The most relevant components are mentioned here. There are two factors in fishes which strongly influence the immune response and antibody production, seasonal changes and temperature. The main lymphoid organs are described here: the thymus, the kidney and the spleen, as well as the lymphocyte ontogenic process. Some advances in knowledge were published during the last years, however it is still necessary to develop new studies in basic immunology in different species to caracterize effector cell populations and their functions.