Cross matching is done by mixing a small sample of recipient blood with a small sample of the donor blood. The mixed blood is looked under a microscope to see if there is any clumping of blood. If there is no clumping it indicates that it is safe to transfuse the donor blood. People must receive blood of the same blood type else serious, sometimes fatal, transfusion reaction can occur. Cross matching is essential for organ transplantation since ABO antigens are also found on most body organs.