c. In RhO and "other" HDN, the antibodies in the mother's plasma result from
previous immunization by transfusion or pregnancy. Exposure does not always result in
immunization, but production of these lgG antibodies almost never occurs without such
exposure.
d. lmmunization commonly results from fetomaternal hemorrhage that occurs at
delivery. Small numbers of fetal RBCs possessing foreign antigens escape through the
placenta and gain access to the mother's circulation, most often during the last half of
pregnancy. In most instances, the small fetomaternal hemorrhage during pregnancy
does not induce immunization.
NOTE:
The term fetomaternal hemorrhage means the leakage of fetal RBCs
into the mother's circulation. The largest fetometernal hemorrhage occurs at
delivery, upon separation of the placenta. Approximately half of all women
have fetal cells in their circulation in the postpartum period. The average
volume of fetomaternal hemorrhage at delivery is less than 1.0 ml of whole
blood; however, some women apparently become immunized by minute
volumes of blood (less than 0.1ml). With volumes of RhO(D)-positive blood
up to 0.4 ml, there is a positive correlation between the frequency of
immunization and the volume of RBCs in the exposure. On rare
occasions, immunization may occur during the first pregnancy by means of
the transplacental passage of fetal cells. This accounts for some failures
observed after prophylaxis with RhO(D), immune globulin, (human), (RhlG).
e. Approximately eight percen of Rh-negative women who have Rh-positive
ABO-compatible babies develop detectable anti-RhO within six months if not protected
with RhlG. An additional eight percent will develop anti-RhO during their next Rh-
positive pregnancy. This is interpreted as a secondary response to a very small
antigenic challenge with the primary immunization occurring at the prior delivery of an
Rh-positive baby, even though detectable anti-RhO could not be demonstrated in the
previous postpartum period.
f. ABO incompatibility between mother and father has the effect of protecting
the mother from Rh immunization. The incidence of Rh immunization is much less
following delivery of an ABO- incompatible child compared with an ABO-compatible
one.
2-15. TRANSFER OF ANTIBODY TO THE FETUS
The fetus becomes passively immunized because the maternal antibody enters
the fetal circulation by placental transfer. Although the exact mechanism of this transfer
is unknown, it is not simple passive diffusion. The transport mechanism is very
selective in that lgG is the only maternal immunoglobulin to cross the placenta. The
fetus is capable of only feeble immunoglobulin synthesis; yet, at birth the fetal plasma
concentration of lgG usually exceeds the maternal plasma lgG concentration. An
increasing rate of transfer is evident during the last few months of pregnancy so that the
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