Rhesus Alloimmunisation

Question 1

From what gestational age does a foetus have red cell antigens?

Answer 1

• 7 weeks

Question 2

How can pregnant women undergo primary immunisation to red cell antigens?

Answer 2

• Blood/transfusion/needle sharing
• Feto-maternal haemorrhage
  
  • Miscarriage
  • Termination
  • Ectopic
  • APH
  • Amniocentesis
  • Version
  • MVA
  • Delivery
  • Occult (idiopathic - increasing frequency and volume with gestation, usually sensitises after 28 weeks)

Question 3

List some red cell antigens that can have important sequelae if present in a pregnant women. Which are the more harmless ones?

Answer 3

• Important
  
  • Rhesus (C/c, D,d, E,e)
  • Kell, Kidd, Duffy,
  • MNS
• Normally harmless
  
  • ABO
  • Lewis
  • P

Question 4

What is the difference between autoimmunity and alloimmunity?

Answer 4

• Autoimmunity
  
  • Mother makes antibody to own antigen
    
    • Thyroid
    • Connective tissue diseases
    • ITP
• Alloimmunity
  
  • Mother makes antigens to antigens she does not have
    
    • Red cell antibodies
    • NAIT

Question 5

How is screening for red-cell antibodies undertaken with a pregnant woman?

Answer 5

• All pregnancies screened for anti-RBC antibodies at first antenatal visit
  
  • First pregnancy - risk stratification by titre
  • Partner grouping - calculating chance that the foetus has Rhesus protein based on father/mother genetics
• Subsequent screening by titre stratified risk

Question 6

How is screening performed for low, medium and high risk pregnant woman in terms of RBC antibodies and their complications?

Answer 6

• Low risk - antibody titre at each visit, deliver at 38 weeks
• Medium risk - titre at each visit, MCA peak flow scan (indicator of anaemia - faster flow), CTG from 32 weeks (sinusoidal trace), deliver at 38 weeks
• High risk - US screening from 17 weeks, foetal blood sampling if MCA PSV increased, intrauterine blood transfusion (irradiated) if anaemic (only at tertiary centre)

Question 7

How is red-cell alloimmunisation prevented?

Answer 7

• Women under 50 have blood transfusion matched to ABO, Rh, and Kell (others difficult to match)
• Passive anti-D
  
  • To be given only within the first 72 hours of a sensitising event
  • Indications
    
    • Administration of anti-D to Rh- women after sensitising events
    • Administration to Rh- women at 28 at 34 weeks
    • Administration to Rh- women post delivery
• Volume
  
  • Small titre (doesn’t affect foetus)
  • Kleihauer if sensitising event (estimated foetal blood in maternal circulation)