Recent studies suggest that transfusion of stored red cell units can sometimes be associated with adverse clinical events including increased risk of cardiovascular complications, infections, and multi-organ failure. The underlying biological mechanisms are not well understood, but appear to involve a variety of changes that occur within the red blood cells and the storage media that cause impaired ability of the red cells to deliver oxygen and promote stimulation of pro-inflammatory pathways in the recipients. In this session, the possible mechanisms of the so-called red cell "storage lesion" will be reviewed and discussed. The session will also review the immunological triggers of stored red cells with focus on the role of iron.