Tests on donated blood. Following a donation, blood is taken to a transfusion centre where it must undergo several tests before being stored in a blood bank. The blood group listed on the unit (seen at left) is checked as a safety measure. A small sample of blood is diluted in test tubes and a drop of the solution is added to other tubes which contain the antibodies (coloured containers at right) of the various blood groups. Agglutination (clumping) occurs if the blood is mixed with a solution which contains antibodies of a different blood group. Donated blood is also tested for hepatitis B virus, syphilis, and antibodies to the HIV virus.