Fetomaternal alloimmunization: an antibody titre variation study

Alloimmunization is the production of antibodies by the mother against non-self antigens located on fetal red blood cells; the involved antigens are over fifty, with the three most common being RhD, Kell, and c. This condition puts the fetus at risk of hemolytic disease of the fetus and newborn (HDFN), characterized by hemolytic anemia and jaundice, whose severity ranges from subclinical to deadly conditions. Currently, the factors determining the severity of the disease remain incompletely understood: with this study, we aimed at characterizing variations in antibody titers and investigating their role in the onset and severity of HDFN. We conducted a retrospective study involving 167 pregnant women presenting anti-D (108), anti-Kell (30) or anti-c (30) antibodies who received care at the ambulatory for fetomaternal alloimmunization of the Sant'Anna Obstetric Gynecological Hospital between 2014 and 2023. Among the 142 infants born at our institution, 37 were healthy, whilst 137 exhibited signs of HDFN. In particular, we registered 52 cases of anemia, of which 26 severe, and 101 cases of jaundice, 13 of which severe. We analyzed variations in the median antibody titers in newborns subdividing them into different groups. First we observed how the median titers for anti-c antibodies were significantly lower than that of the other two, ranging from 1/8 to 1/32; in contrast, the median titer ranged from 1/32 and 1/288 for anti-D, and from 1/64 to 1/272 for anti-Kell antibodies. We also noted how affected infants had higher median titers as compared to healthy ones, both in general and concerning anemia and icterus specifically. Another relevant finding is that the median antibody titer rose from weeks 25-28 to weeks 29-32 in all the groups. Such increase can be attributed to the augmented fetomaternal blood exchanges as pregnancy progresses. We then evaluated the adequacy of the critical titers utilized at our institution, which define the threshold above which the fetus is at risk of developing severe HDFN. In our study, we chose a critical titer of 1/16 for anti-D and anti-c antibodies and of 1/4 for anti-Kell; this topic is still a matter of debate and other institutions employ higher values of 1/32 and 1/8 respectively. Our selected critical titers exhibited a sensitivity of 100% for severe HDFN and of 96% for HDFN of all grades. Our secondary objective was to investigate the maternal anamnesis to determine the triggering event for immunization. Out of the 161 women for whom anamnesis was available, 148 (92%) had at least one previous pregnancy, which can be considered the immunizing event. Among the 13 primigravidae, 8 had previous abortions and/or blood transfusions that could explain the immunization, while for 5 the anamnesis was silent. It is possible that a fetomaternal hemorrhage occurred early during that pregnancy, or that those women became pregnant in the past and had an early miscarriage before noticing the pregnancy. Our findings contribute to the existing literature by investigating the trajectory of antibody titers throughout pregnancy and their relationship to neonatal outcomes, a topic that has received limited attention in previous studies. Our work highlights the significance of antibody titers, but other factors such as maternal antibody subclass and glycosylation, efficiency of transplacental IgG transport, functional maturity of the fetal spleen, presence of HLA-related inhibitory antibodies, and fetal sex may also play substantial roles, warranting further investigation.