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The RhD antigen is a protein found on red blood cells in humans - VCE - SSCE Biology - Question 5 - 2014 - Paper 1
Question 5
The RhD antigen is a protein found on red blood cells in humans. The presence of the protein is genetically controlled. Possible phenotypes and genotypes with respec... show full transcript
Worked Solution & Example Answer:The RhD antigen is a protein found on red blood cells in humans - VCE - SSCE Biology - Question 5 - 2014 - Paper 1
Step 1
With reference to the father's possible phenotyped(s) and genotype(s), explain how a Rhesus-negative mother could be pregnant with a Rhesus-positive fetus.
Answer
The father could be either a homozygous Rhesus positive (DD) or heterozygous Rhesus positive (Dd). If the father is DD, all offspring would inherit a D allele, resulting in Rhesus-positive phenotypes. If the father is Dd, there is a 50% chance of the offspring being Rhesus positive (Dd) and a 50% chance of being Rhesus negative (dd). Therefore, it is possible for a Rhesus-negative mother (dd) to conceive a Rhesus-positive child if the father contributes a dominant D allele.
Step 2
Describe the risk of hemolytic disease of the newborn (HDN) can occur if a Rhesus-negative mother is pregnant with a Rhesus-positive fetus.
Answer
Hemolytic disease of the newborn (HDN) can occur because the Rhesus-negative mother produces antibodies against the RhD antigen present in the Rhesus-positive fetus. When fetal blood cells enter the mother's circulation during pregnancy or at birth, the mother’s immune system may recognize the RhD antigen as foreign and mount an immune response, producing IgG antibodies. In subsequent pregnancies, these antibodies can cross the placenta and attack fetal red blood cells if the fetus is again Rhesus-positive, leading to anemia, jaundice, or even fetal death.
Step 3
Explain, by referring to the immune response, why the fetus in a second pregnancy is at far greater risk of HDN than the first Rhesus-positive fetus.
Answer
In the first pregnancy, the Rhesus-negative mother is exposed to the fetal RhD antigen, but her immune system does not produce enough antibodies to cause harm to the fetus. However, upon exposure, memory B cells generate IgG antibodies, which can persist. In a second pregnancy with another Rhesus-positive fetus, the presensitized immune system quickly produces large amounts of IgG antibodies that target the fetal RhD-positive red blood cells, leading to a higher risk of HDN. This heightened immune response significantly increases the severity of the disease compared to the first pregnancy.