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HELP why does HDFN / HDN only occur in subsequent pregnancies and not the first?

*medical student question*

so I was thinking along the lines of in the first preg, the mother wont have any memory b cells and thus these won't clonally expand and differentiate to produce IgG anti-D antibodies - since its first exposure. But wont the immune system eventually recognise the foreign D antigens in the mother's blood and form the anti-D antibodies that'll cross the placenta and cause haemolysis of the fetus' red blood cells?
Original post by starrystudy
*medical student question*

so I was thinking along the lines of in the first preg, the mother wont have any memory b cells and thus these won't clonally expand and differentiate to produce IgG anti-D antibodies - since its first exposure. But wont the immune system eventually recognise the foreign D antigens in the mother's blood and form the anti-D antibodies that'll cross the placenta and cause haemolysis of the fetus' red blood cells?


The maternal and foetal circulations do not mix unless a sensitising event has occurred - the most common sensitising event is during childbirth when foetal RBCs may enter the maternal circulation and stimulate the production of anti-RhD antibodies. However, by the time these antibodies have been generated, the baby would have been born. In a subsequent pregnancy with a RhD+ foetus, if foetal cells enter the maternal circulation it will stimulate memory B cells to produce IgG anti-RhD antibodies which cross the placenta, and as you will know the secondary immune response is much more vigorous than the primary immune response.

It is possible that during the first pregnancy with a RhD+ foetus, an early sensitising event (e.g. amniocentesis) will stimulate the primary immune response. If there is a second sensitising event during the same pregnancy, I see no reason why the first baby could not develop HDN, however, it is far less common as this would require two sensitising events.

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