Transfusions are not without risk. One of the side effects of transfusions is the development of transfusion-induced immunomodulation (TRIM)--primarily immunosuppression, but also a strong proinflammatory effect. This may be the cause of acute lung injury (TRALI), multiorgan failure (MOF), transfusion related acute-graft-versus-host-disease (TR AGvHD), as well as of the development of secondary nosocomial infections, mostly pulmonary infections, sepsis and wound infections, and also of elevated number of tumour relapses in oncological patients. The causes of TRIM development are the induction of microchimerism, different cells and also soluble factors--complement components, such as C3a, soluble HLA-I and HLA-II molecules (HLA--human leukocyte antigen), soluble Fas ligand (sFasL), and others. The immunosuppressive potential of blood products grows with the time of their storage and becomes highest in non-leukoreduced blood products stored for a long time. In view of possible adverse effects of a transfusion, the expected benefit should be balanced against possible risks.