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