A blood transfusion was the transfer of blood or blood components from one lifeform to another. Compatibility issues between the blood donor and recipient could occur due to differences in antigens or antibodies, specialized proteins used to fight foreign bodies such as viruses or bacteria. It was also likely that interspecies incompatibilities would occur due to more fundamental differences in blood chemistry; for example, Human hemoglobin was iron-based, whereas Vulcan hemoglobin was copper-based.
Bolians were unable to accept direct transfusions from Vulcans; doing so would result in the death of the Bolian. However, there were instances where incompatible blood was altered in order to make a transfusion possible. (VOY: "Prototype")