The most important factors are that the recipient not have produced isohemagglutinins, and that they have low levels of T cell-independent antigens. UNOS regulations allow for ABOi transplantation in children under two years of age if isohemagglutinin titers are 1:4 or below, and if there is no matching ABOc recipient. Studies have shown that the period under which a recipient may undergo ABOi transplantation may be prolonged by exposure to nonself A and B antigens. Furthermore, should the recipient (for example, type B-positive with a type AB-positive graft) require eventual retransplantation, the recipient may receive a new organ of either blood type.