OB: Alloimmunization Flashcards
a process by which a woman develops antibodies against antigens on the red blood cells of her fetus.
Alloimmunization (also isoimmunization)
What kind of antibodies develope from the mom in alloimmunization?
exposure to a foreign antigen, development of IgM then IgG-secreting plasma cells, and memory cells that sustain immunity against the red blood cells expressing the foreign antigen.
What antigen on RBCs is the major one that has active management and prophylaxis?
Rhesus Antigen
Why is it that a G1P0 mom who is Rh- with an Rh+ fetus have an immunization reaction that goes unoticed?
IgM CANNOT cross the placenta
Throughout the pregnancy, the syncytiotrophoblasts of the chorion/placenta prevent the mixing of maternal and fetal circulations while also preventing maternal immune cells from accessing any fetal antigens. Then, some form of maternal/fetal blood mixing (usually delivery) occurs. Fetal red blood cells are exposed to the maternal bloodstream. The Rh antigen is recognized as foreign. Immunity happens. IgM is developed against the Rh antigen. If this first fetus is still gestating, then IgM cannot cross the placenta, so it cannot harm the fetus. Because the mixing of maternal and fetal blood occurs most commonly during delivery, the first exposure typically goes unnoticed entirely. Immunity continues; IgG-secreting plasma cells release anti-Rh IgG antibodies, and memory cells sensitized to the Rh antigen lie in wait for the next exposure. The woman is now said to be sensitized to the Rh antigen
Why is it that a woman who is Rh- who is sensitized to Rh+ can mount an immune response against her fetus?
IgG CAN cross the placenta, this can lead to fetal anemia. The more sensitized a woman is to Rh+, the worse the immune reponse.
Five criteria must be met to consider fetal anemia. What are these criteria?
Fetal Risk (all five positive)
1. Mom is Rh-antigen negative
2. Dad is Rh-antigen positive or unknown*
3. Mom is anti-Rh-antibody positive
4. Antibodies cause anemia (Rh antibody, others)
5. Critical titers (≥ 1:16 for anti-Rh)
- If there’s any question of paternity, then cell-free DNA testing can be performed to identify the fetal genotype. This was formerly done by amniocentesis, but with the advent of cell-free DNA testing, amniocentesis is no longer worth the risk.
What titer ratio is the cutoff for risk of fetal anemia?
If the anti-Rh titer is 1:16 or greater (the bigger the number after the colon, the more positive the titer), there is a risk of fetal anemia.
When do we do sequential titers of anti-rh antibodies from mom throughout the pregnancy?
If she is Rh− and anti-Rh-antibody negative, prophylaxis is the only concern (see Preventing Alloimmunization, below). If she is Rh− and anti-Rh-antibody positive, then antibody titers are assessed.
How do we screen for fetal anemia in utero?
Assess for fetal anemia with an ultrasoundDoppler of the middle cerebral artery. Oxygen delivery is dependent on cardiac output, hemoglobin, and percent saturation of hemoglobin by oxygen. With a decrease in fetal hemoglobin, there is an increase in cardiac output. Increased velocity in the middle cerebral artery of the fetus indicates fetal anemia.
Note, Doppler of the MCA is the standard. Amniocentesis and Liley graph are always the wrong answer and never the best next step
How do we confirm fetal anemia in utero?
Percutaneous umbilical cord sampling (PUBS, aka cordocentesis) obtains a sample of fetal blood to assess fetal hemoglobin. It also provides a route for transfusion. It is rarely done because of the high risk of fetal loss. The decision to do PUBS and transfusion vs. to deliver is based on gestational age, not fetal hemoglobin.
If gestational age < 34 weeks, do PUBS and transfuse.If gestational age ≥ 34 weeks, deliver.
When do we give anti-D immune globulin?
At 28 weeks, all Rh− patients with a negative antibody screen should be given a standard dose (300 μg) of anti-D immune globulin
anti-D immune globulin should be administered within 72 hours after delivery.
it is also indicated after any opportunity of mixing of maternal and fetal circulations:
Spontaneous or induced abortion
Ectopic pregnancy
Invasive procedures such as amniocentesis and chorionic villus sampling
External cephalic version
Antenatal vaginal bleeding after 20 weeks
Abdominal trauma with hemorrhage
What other antibodies do we have to worry about?
Lewis antibodies Live (no effect on baby). Kell and Kidd antibodies Kill (kills fetus)Duffy and Diego antibodies Die (fetus dies).
