Free radicals contribute significantly in modification of immune processes and inflammatory reactions. They are produced by activated phagocytes which use them for killing microorganisms. Free radicals facilitate production of cytokines, which are important as modifiers of inflammatory reactions. Formation of free radicals is influenced by antioxidants which can thus modify the intensity of inflammatory reaction and immune response. The authors describe in detail the contribution of free radicals in etiology and pathogenesis of autoimmune diseases including rheumatoid arthritis, multiple sclerosis or amyotrophic lateral sclerosis. The role of free radicals and modifying influence of antioxidants in viral, bacterial, parasitic and mycotic diseases is described in the second part of the review. Finally, influence of free radicals and antioxidants on immunity changes in patients with malignant tumours, during aging and physical exercise is discussed.