Background:Alterations of the immune systemin obese individuals have been well documented. Studies have shown that peripheral blood mononuclear cells (PBMC) are closely associated with the tendency to inflammation and infections observed in obese, otherwise healthy persons, in part because of their increased capability to produce inflammatory cytokines. The aim of the study was to elucidate the question if this phenomenon is innate or it is acquired during their contact with the fatty tissue and/or abnormal substances in the serum of obese individuals. Methods: PBMC from normal-weight individuals were incubated for 24 hours with increasing concentrations of sera from obese subjects and from healthy normal weight individuals their capacity for inflammatory cytokines production was detected and compared. Results: While the effect of low concentrations of sera from obese and normal weight individuals on cytokine production by normal PBMC was similar, higher concentrations induced increased secretion of the pro-inflammatory cytokine IL-1â, and a decreased production of the ant-inflammatory cytokine IL-10. The secretion of the remaining cytokines examined was not affected. Conclusions: This observation suggests that the capability PBMCfromobese patients to produce increased levels of pro-inflammatory cytokines is probably acquired.