I have a lot of request from teachers/students about how to explain the rhesus factors in mother/embryo in 1st & 2nd birth. This post is my reply:
First, the question would be something like this:
A mother has Rh(-). Explain why her 1st-born Rh(+) baby has no problem but the 2nd-born Rh(+) baby does.
For the 1st born, look at the diagram below:
As you can see, mother is Rh(-) and the baby indeed Rh(+). Let's use Rh(-) as the 'bad guy' (呆子) and Rh(+) as the 'good guy' (善良的人). As usual the bad guy will attack the good guy. But here, nothing happen because mother's immune system has yet to produce 'weapon' (antibody) to attack the baby's 'target' (antigen). In other words, mother's immune system is simply not ready.
Now, let's see what happen in the 2nd-born baby with similar Rhesus factor:
You realize the bad guy (mother) already produced the 'weapons'. This time, the mother's immune system is ready with full antibody. As a result, the baby's Rh(+) is attacked, causing harm to the baby.
This condition is known as erythroblastosis fetalis.
It killed most babies before the Industrial Ages. Thanks to the modern medicine, we have ways to prevent this from happening...
Congratulations! Now this topic poses no hindrance to you. We shall meet later near the 2014 SPM exam to discuss the right techniques to answer this type of questions -and tonnes of fun- during my big seminar this year in FRIM. Wish you well in your study! :-)