chapter 8, part 1

                                                      Chapter 8. 

Immune Systems in the Pathogenesis of  RhD-Hemolytic Disease of Fetuses and Newborns

In previous chapters, we showed that fetal cells occasionally enter the maternal circulatory system, as reflected by ensuring  documentation of medical tragedy. When the fetus has the red blood cell D antigen of the Rh group (the fetus is Rh-positive) and the mother does not (she is Rh-negative), the entry of fetal blood cells into the mother's blood will cause an immune response against the antigen. The mother produces antibodies against the D antigen, and these may cross the placenta and start destroying the fetal red blood cells: a condition called hemolytic disease of the newborn (HDN).

This disease was first described by a French midwife in 1609, in a set of twins: the first twin was hydropic and stillborn, and the second was deeply jaundiced and subsequently died of what we now call kernicterus (cit. after 1). The two conditions were not associated again until 1932, when Diamond et al. (2) demonstrated that hydrops and kernicterus were two aspects of the same disease in which hemolysis of the red cells of fetuses and neonates results in extramedullary erythropoiesis, causing hepatosplenomegaly and an outpouring of erythroblasts into the circulation, a condition they termed erythroblastosis fetalis.

The identification of the cause of the hemolysis had to await the discovery of the Rh system in 1940 and the determination, shortly thereafter, that hemolytic disease of the fetus had occurred in an RhD-positive fetus carried by an RhD-negative woman who had been immunized by transplacental passage of RhD-positive red cells during a prior pregnancy (3). Maternal IgG antibodies to RhD traverse the placenta, coating and destroying the RhD-positive fetal red cells and initiating the chain of events that leads to death from fetalis hydrops in 25% of affected fetuses and death from kernicterus in 25% of affected neonates.

Despite the fact that great strides have been made in the ability to determine fetal RhD status, and in  preventing and managing of RhD isoimmunization (4-7), HDN remains a great problem in obstetrics and pediatrics (8). The maternal reaction to the RhD antigen is one of strongest and most common maternal alloimmune reactions (1). Although the use of RhD-immunoprevention over the last three decades has made a substantial contribution to the decline of RhD-HDN, RhD sensitization continues to occur in RhD-negative women at a rate 30% to 40% when they are bearing an RhD- positive child (9,10). Despite the availability of an effective preventive measure (11,12), RhD-HDN continues to contribute significantly to infant morbidity and mortality (13).  Until recently, ultrasound examination, amniocentesis, and fetal blood sampling are, in many cases, the only tools available to assess the status of fetuses and the risk for hemolytic disease in RhD-sensitive women (14,15). The application of genetic determination is only now taking its first steps towards clinical use  (5,16-18).

Modern immunology added new  approaches for a better understanding of the pathogenesis of this disease. HDN is no longer considered to result erythrocyte lysis with its attendant consequences, such as hyperbilirubinemia, anemia and compensatory erythroblastosis (1,19). When an RhD-negative mother is exposed to the RhD-positive red cells (usually by transplacental haemorrhage), she develops allo-anti-D which crosses the placenta and then results in the destruction of fetal red cells. Clinical manifestations of HDN range from asymptomatic mild anemia to hydrops fetalis or stillbirth associated with severe anemia, hyperbilirubinemia and jaundice (20).

Unfortunately, little attention has been paid to the role of the immune system in fetuses and newborns, which are capable of producing an immune response during the second half of the gestational period (21). The fact that Igs are themselves antigens needs to be taken into account. As Rh IgGs, they can induce an immune reaction via the formation of anti-Ig antibodies    (22-24). The possibility of immune complex (IC) formation during HDN, such formation having been seen in different infectious diseases  of fetuses and newborns (25,26), and a role for these complexes in the pathogenesis of this disease remain unclear.  

The mechanism of erythrocyte destruction has been studied in the icteric form of HDN but it has not been proven in the hydropic form of the disease. There is no theory to explain the appearance of different forms of HDN which agrees with the modern conception of pathology and immunology of fetuses, newborns and the placenta. Our knowledge regarding the hydropic form of HDN is scant, and this is one of the reasons for the negative results obtained during the course of its prevention and treatment   (10-12). These problems are discussed in more detail in this chapter.

8.1) The immune response of fetuses and newborns in RhD conflict

The immunocompetence of fetuses during the second half of gestation and of newborns has been described in many publications (27-30).  The  immune response of fetuses and newborns is different from that of children and adults. Fetuses synthesize a large amount of IgM together with IgA (31-33) and IgE (34-36)  as a manifestation of their immune response to antigenic attack. The synthesis  of IgA, IgG and IgM by thymic B cells has been shown in pig fetuses (37).

The fetal immune  response to antigenic attacks has its own unique morphological  features (38,39). Many T and B lymphocytes are transformed into  lymphoblasts, and there is an increase in the number of IgM-synthesizing cells and macrophages (40-42). The reactive centers in the spleen and lymph-node follicles and the mature plasma cells are not formed in fetuses and newborns (43,44), as they are in children and adults (45,46).

The blood level of IgG in infants with any form of icteric HDN (with and without anemia) has been found to vary between 1241±92 and 1395±115 mg/100 ml compared to 12.2±3.3 mg/100 ml in healthy newborns   (47). Blood levels of the other types of Igs (IgM and IgA) were also  significantly higher in infants with icteric HDN. In infants with the hydropic form of HDN,  the blood level of IgG was also higher compared to healthy newborns, but the blood levels of IgA and IgM were significantly lower (Fig. 15). 

Morphologically, icterus without anemia is characterized by a significant increase in the number of lymphoblasts (B and T lymphocytes) in the spleen and lymph nodes: from 2.0±0.8 and 2.4±0.2 in controls to 10.6±3.4 and 10.8±1.6 in the ill infants (47).  The number of IgM-positive cells, macrophages and siderophages also increases significantly. In the thymus, features of accidental involution in the weight and area of the cortex are seen. Extramedullar erythroblastosis is low or nonexistent in the spleen, liver and other organs.

In cases of icterus with anemia, the morphological changes of the spleen, lymph nodes and thymus are more distinct (47).  The number of IgM-positive cells increases significantly whereas there are only a few solitary IgA-positive cells. B and T lymphocytes and IgM-positive cells are accumulated not only in the spleen and lymph nodes, but also in the liver and lungs. The number of follicles in the spleen per 10,000 μm² of slide section decreases, as does their area. The area of the lymph-node parenchyma also decreases as a result of the lower total number of lymphoid cells. Erythroblastosis is high in the liver and spleen. In the thymus, phase III of accidental involution (AI) is found.

The hydropic form of HDN, seen in  60% to 80% of the cases in immature 20- to 32-week-old fetuses (47), is characterized by a significant decrease in the total number of all types of lymphocytes, IgM-positive cells and macrophages. The follicles in the spleen disappear or are present as solitary groups of lymphocytes. In the red pulp of the spleen and in the liver, the number of nuclear erythrocytes, especially erythroblasts, increases significantly. Lymph nodes exhibit  signs of devastation: sinuses are wide, containing single macrophages without phagocytic features. The parenchyma contains only stromal cells and a small amount of lymphocytes. The thymus shows phases III-IV of AI with an increased number and area of thymic corpuscles, which sometimes form cysts.

Although both variations of the icteric form of HDN (with and without anemia) exibit great similarity in their immune-response char­acteristics, the icteric form with anemia exhibits those char­acteristics to a more salient degree. However, this form of HDN exhibits signs of immune-insufficiency. There is a decrease in the total number of T- and B-lymphocytes and lymphoblasts per 10,000 μm² of spleen area, in the number of follicles and their total area, as well as in the areas of the lymph-node parenchyma and of the thymus cortex decreased (47). Concomitant to these features, the number of cells decreases to 30% to 50% of their number in healthy newborns.

Fig. 15.  The serum levels of Igs in newborns with different forms of RhD disease (% of  healthy newborns) 

Groups of infants: A, icteric form without anemia; B, icteric form with anemia;

                              C, hydropic form.