Alloimmunization

Question 1

What is the mechanism of action of RhIG?

Answer 1

RhIG is anti-D IgG
Binds fetal red blood cells in the maternal circulation, preventing the maternal immune system from recognizing & reacting to them
Crosses the placenta & binds fetal red blood cells in the fetal circulation, but does not cause hemolysis

Question 2

How much RhIG is needed to protect against 1 mL fetal blood (0.5 mL RBCs)?

Answer 2

10 mcg RhIG per mL fetal blood

Question 3

Why does ABO incompatibility protect against Rh alloimmunization?

Answer 3

ABO incompatibility results in rapid hemolysis of fetal RBCs in the maternal circulation; there’s no time for the maternal immune system to recognize & mount a response to the Rh antigens

Question 4

If a woman receives routine RhIG at 28 weeks, then delivers a D-positive infant at 30 weeks, does she need an additional dose of RhIG?

Answer 4

120 mcg RhIG can be given, provided there is no indication of excessive feto-maternal hemorrhage

Question 5

At what gestational age is the D antigen detectable on fetal RBCs?

Answer 5

7+3

therefore no need for prophylaxis for SA/TA prior to this gestational age

Question 6

Should a D-negative woman receive prophylaxis after evacuation of a molar pregnancy?

Answer 6

Complete mole - minimal risk, no expression of D antigen

Partial mole - potential risk, give RhIG unless the diagnosis of complete mole is absolutely certain