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Red blood cell ( RBC) antibodies can persist for decades or decrease quickly to undetectable levels. Antibody persistence has not been systematically studied. Women whose children are treated with intrauterine transfusions ( IUT) for haemolytic disease of the fetus ( HDFN) often produce additional antibodies, which can be evoked by the intrauterine transfusion or by fetomaternal haemorrhage during the procedure. Factors associated with persistence of both the antibodies responsible for HDFN and additional antibodies were studied in 260 women whose children were treated with IUT between 1988 and 2008. They possessed 499 (205 anti-D and 294 non-D) antibodies after the last IUT. After a median follow-up of 8·7 years, all 260 antibodies primarily responsible for HDFN had persisted. Additional antibodies directed against antigens of the children persisted in 70·6%, and in 32·3% if they were not child-specific ( P < 0·001). Antibodies induced by irradiated IUT persisted in only 7·1%. Multivariate analyses showed that non- HDFN antibody persistence was dependent on the antibody titre and specificity. In conclusion, persistence of antibodies mainly depends on antibody strength and specificity. Difference between fetal or non-fetal immunogens suggests maintenance of antigenic stimulation possibly by long-term fetomaternal chimerism. [ABSTRACT FROM AUTHOR]