A crossmatch is performed prior to administration of blood or blood products (e.g. packed red blood cells). The purpose of the crossmatch is to detect the presence of antibodies in the recipient against the red blood cells of the donor.  To complete this test a sample of blood is taken from the donor and recipient.  In the laboratory, the donor blood cells are mixed with the recipient serum. Only those with a negative crossmatch can proceed with the donor evaluation. A negative reaction means that the recipient does not have antibodies against the donor HLA and a transplant can be performed. If the recipient serum kills off the donor cells, this is a positive crossmatch and a transplant would not survive.