Allogeneic blood transfusions alter recipient biology, particularly the immune system, in complex and often deleterious ways. Favorable effects are counterbalanced by evidence for increased postoperative infections, alloimmunization and tumor progression/recurrence. Adverse outcomes with blood transfusion often occur in a dose-dependent fashion. In this session, potential mechanisms and supporting data for these clinical and biologic effects will be discussed. The paradigms of alloimmunization and transfusion immunomodulation (TRIM) need to expand to include organ-specific dysfunction due to inflammation (e.g., TRALI/MODS), thrombosis (e.g., myocardial infarction, stroke), and tumor growth with increased metastatic potential.