A crossmatch also has limitations:
Will not detect errors in patient identification (unless a previous record exists)Will not detect ABO mix-ups if blood types are compatible (can crossmatch group A donor blood for an AB recipient)Will not detect Rh errors (can crossmatch Rh+ donor blood with Rh negative recipient with no reaction if the patient has no anti-D)Will not detect all recipient antibodies to donor antigens (antibody may be too weak to detect, but still cause transfusion reaction such as the Kidd antibodies)Will not prevent alloimmunization of recipient (only ABO and Rh antigens matched - patient can potentially make antibody to all the other antigens) This is why many of the discovered antibodies are found in multi-transfused patients.