In the present work, the phenomenon of bystander effects (transfer of damaging factors from affected cells to intact cells) induced in vivo has been studied. A newly developed method allows evaluation of the effects of bystander factors from sera of different groups of populations on immortalized culture of human keratinocytes. This method has serious advantages as compared to that which was used earlier. The level of damaging bystander factors in blood of people irradiated by the Chernobyl accident was estimated. Affected population groups included liquidators of the Chernobyl accident and people living and working in areas of Gomel region contaminated by radionuclides. The data clearly shows that blood serum samples of irradiated populations contain some kind of factors, induced in vivo and able to cause cell damage when transferred to in vitro conditions. These factors are able to circulate in blood stream for more than 20 years.

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