The effects of pollution on the immune response of fish are important in the dynamics of host/parasite interactions. Although there has been no definitive study that correlates a pollutant with the change in immune status of the fish and subsequent susceptibility to parasitic infection, there have been investigations which have noted changes in several immune parameters. The variable effects of pollutants on innate immunity, antibody levels and leucocyte numbers, migration and activity are reviewed and the consequences of observed changes for parasite fauna assessed. Studies on the exposure of C. carpio to cadmium have revealed an increase in apoptosis in pronephric cells. The implication of this immunosuppression is discussed.