Found this to expain bit better :::
The Rh factor is also known as the D antigen. The baby will receive one gene from each mom and dad. If either of those genes is the Rh + gene the baby will be Rh +. In your families' case, your wife is Rh neg. so she has two copies of the Rh negative gene. With you being Rh + and having a baby that is Rh neg. you must have one copy of the + gene and one of the neg. gene (causing you to be Rh + but able to pass the Rh neg. gene to your child).
When mom's are Rh negative there is a chance that the baby could be Rh +. In this case, if some of the baby's blood gets into mom's blood stream she may develop antibodies to the Rh+ blood cells. These antibodies subsequently can cross the placenta and destroy the baby's blood cells causing severe problems.
To prevent this, Rh negative moms are given an injection of Rhogam. This is Rh + antibody. While it may seem counter-intuitive to give the antibody that we are trying to prevent from forming the reason is quite clever. This way, there is some, but not much, of the antibody floating through mom's body. If any of the Rh + baby blood cells get into the mom's blood stream the antibody binds them before mom's body is able to react so that mom never forms antibodies to the red cells.
If anti bodies are however formed in the mom for some reason then Rhogam is no longer useful and future pregnancies will need to be closely monitored.