HDFN

Common HDFN Antibodies

A/B

D

Rh

Kell

Rare HDFN Antibodies

Duffy

Ss

Kidd

MN

Critical Titer for Anti-D

1:16 and higher

Critical Titer for Anti-K

1:8 or higher

Criteria for receiving RhoGAM

- Rh negative Mom/potential Rh pos baby

- no history of Anti-D

Kleihauer-Betke Calculation

# fetal cells / # maternal cells = % bleed

RhoGAM Dosage Calculation

(% bleed x 5,000mL) / 30mL = dose vials + 1

The 5,000 mL/30mL is the maternal volume

Fetal Neonatal Alloimmune Thrombocytopenia

IgG platelet antibodies cross placenta

80% caused by Anti-HPA-a1

Immune Thrombocytopenic Purpura (ITP)

Autoimmune disorder in mother that can also affect the fetus

15% of neonates need platelet transfusion at birth

Why RhIg (RhoGAM) is only given to Rh-negative mothers

1. Mechanism-specific – RhIg contains only anti-D; it neutralizes fetal D+ RBCs in the maternal circulation. It has no effect on other Rh antigens (C, c, E, e) or non-Rh antigens.

2. Highest immunogenicity – D antigen is the most immunogenic RBC antigen after ABO, with up to 80% of D− individuals producing anti-D after a single exposure. It causes severe, often life-threatening HDFN.

3. High population prevalence – About 80–85% of the population is D+, so an Rh-negative mother has a high probability of carrying a D-positive fetus, making the risk clinically significant.