Testing blood compatibility. Pipette being used to add a blood mixture to a centrifuge tube to test the compatibility of donor and patient blood. This is known as crossmatching. The blood groups of the donor and patient blood will have been determined, but this is a final check to ensure that no other, potentially fatal, immune system reactions occur. The mixture contains both donor and patient blood, and an adverse reaction will cause the red blood cells to clump (agglutinate). When the sample tube is spun in a centrifuge, the gel (pale blue) will filter out any clumped cells. The three tubes show samples that passed through the gel because clumps did not form and the blood is safe to use.