The paper presents the current knowledge about the role of the trace element selenium in the immune system, and possible mechanisms of its action during the immune response. Investigations to date, based on animal and human models, have delivered increasing evidence of Se involvement in the functioning of the immune system. Recently, the importance of selenium in the progression of some viral diseases (e.g. AIDS) has been revealed. However, several questions remain to be explained, such as the turnover of selenium compounds in the cells of the immune system, since no complete theoretical framework yet exists at the molecular level. The problem of the influence of dietary selenium (included in the normal diet, or a diet only slightly modified with selenium) on human immune functions also needs to be resolved.