Currently, islet transplantation as a cell therapeutic option for type 1 diabetes occurs via islet injection into the portal vein. Direct contact between islets and blood is a pathophysiological “provocation” that results in the instant blood-mediated inflammatory reaction (IBMIR) and is associated with early islet loss. However, the nature of the various insults on the islets in the blood stream remains mostly unknown. To gain insight into the mechanisms, we utilized a simplified in vitro model in which islets were exposed to blood in different clinically relevant but increasingly challenging, autologous, allogeneic, and xenogeneic combinations. Irrespective of the blood type and species compatibility, islets triggered blood clotting. Islet damage was worse as islet, and blood compatibility diminished, with substantial islet injury after exposure of porcine islets to human blood. Islet damage involved membrane leakage, antibody deposition, complement activation, positive staining for the membrane attack complex, and mitochondrial dysfunction. Islet damage occurred even after exposure to plasma only, and specific complement inactivation and neutralization of IgM substantially prevented islet damage, indicating the importance of humoral immunity. Efficacious measures are needed to reduce this injury, especially in view of a potential clinical use of porcine islets to treat diabetes.