Final answer:
The correct procedure when one unit is incompatible during cross-matching is to use only the units that are compatible. Incompatible units can cause transfusion reactions, so additional compatible units should be sought through further cross-matching.
Step-by-step explanation:
When one cross-matched unit out of five is incompatible at the antiglobulin phase, the procedure that should be followed is to use the compatible units only. Incompatible units suggest the presence of antibodies that might react with the donor red blood cells, leading to a transfusion reaction. To ensure patient safety, only the units that have shown compatibility during the cross-match test should be used for transfusion. Further cross-matching with a different set of units should be performed to find additional compatible units if necessary.