How safe is blood, really?

Blood is safer than it has ever been, however the progression of transfusion from dangerous intervention to reliable supportive care been non-linear. Disparities resulting from geography, economy, and social class persist. Some risks are known, others are unknown but predictable, and still others may be totally unpredictable. Among the known risks are infectious and immunologic events that can be calculated per unit of blood transfused. These risks vary by component. Among the unknown risks are the potential for emerging pathogens transmitted by blood and for processing or storage lesions to result in short or long-term toxicity. National registries provide some reassurance that transfusion may not affect mortality significantly beyond the first few weeks after administration. Nevertheless, transmission of novel pathogens, repeated allogeneic stimulation, and infusion of cytokines or chemokines may have unrecognized consequences. Blood safety can be effected dramatically with small investment in developing countries. In the developed world, technologies such as pathogen inactivation, antigen camouflage, component substitutes, or cell expansion promise relatively small advances in safety at substantial cost. No strategy guarantees zero-risk.