Rh sensitization occurs when in subsequent pregnancies maternal antibodies attack fetal red blood cells. Which statement best describes this process?

• A. The paternal blood enters the maternal circulation causing antibodies
• B. The fetal blood can enter the mother's circulation and produce maternal antibodies against the Rh factor
• C. Maternal antibodies are produced in response to maternal Rh antigen
• D. Rh-negative mothers never react in future pregnancies

Answer: (B)

Rh sensitization happens when the mother’s immune system is exposed to the Rh (D) antigen on fetal red blood cells. During an initial pregnancy with an Rh-positive fetus, fetal RBCs can enter the mother’s circulation, especially around delivery or other events, and the maternal immune system recognizes the D antigen as foreign. This prompts production of IgG anti-D antibodies. In a subsequent pregnancy, if the fetus is Rh-positive, these antibodies cross the placenta and attack the fetal red cells, leading to hemolytic disease of the fetus and newborn.

The statement that best describes this process is that fetal blood can enter the mother’s circulation and provoke maternal antibodies against the Rh factor. The other options don’t fit as well: paternal blood crossing into the mother isn’t the mechanism, antibodies are formed in response to fetal (not maternal) Rh antigen, and Rh-negative mothers can indeed develop antibodies and be affected in future pregnancies.