MadSci Network: Immunology

Re: How the RhoGam works?

Date: Wed Dec 16 12:15:21 1998
Posted By: David Pendergrass, Faculty, Basic Medical Science, University of Missouri - Kansas City School of Medicine
Area of science: Immunology
ID: 913628829.Im


  Good question!  

  Ok, first let's review.  As you know, a woman who is Rh negative means 
that she does not have what is called the "D antigen" on her red blood 
cells.  Some people call the "D antigen" the Rh antigen.  An antigen is 
a molecule on the surface of the red blood cell that can cause antibodies 
to form.  When you hear someone say AB pos or O neg, the positive means 
that they have the Rh antigen and the negative means they don't have the Rh 
antigen.  Ok, so far?

  Now, if you are Rh negative, you not only don't have the antigen, but you 
MIGHT make antibodies to Rh positive blood.  Normally, though, women with 
Rh negative blood don't get exposed to Rh positive blood, except when they 
are pregnant!  If her child is Rh positive and she is exposed to her 
baby's blood when the baby is delivered, then she will make new antibodies 
to Rh positive antigens.  Not a problem for her first baby!

  But what about a second baby?? Well, if the baby is Rh negative, then her 
new antibodies can't hurt a thing, because the baby doesn't have the Rh 
antigen!  But, if her baby is Rh positive, then watch out!  Because then 
her new Rh antibodies can attack the red blood cells of the baby, causing a 
condition called erythroblastosis fetalis.  Not good for the second Rh 
positive baby.

  Ok, enough review!  What about RhoGAM?  If you look again, you will see 
that this whole scene occurs because the Rh negative mother develops 
antibodies to Rh antigens.  It's the new antibodies that's the problem.  
So, what RhoGAM does is prevent the antibodies from forming in the first 

  And here's how it does it.  Immediately before or after each delivery, 
the obstetrician will administer RhoGAM (that answers one of your 
questions!).  RhoGAM is actually antibodies itself!  The RhoGAM antibodies 
then attach to the Rh positive red blood cells of the baby(if it has any 
Rh antigen).  This covers the Rh antigen, so that the mother's immune 
system cannot make antibodies to it!  So, since the mother doesn't make 
antibodies, she can't hurt the second baby.  Of course, RhoGAM has to be 
given at each birth so that she doesn't hurt the next child.

  For a reference, see the following:

Anatomy & Physiology, 2nd edition, Seeley, Stephens, Tate, 
Moseby-Year Book, Inc., ISBN 0-8016-4832-7, p.602-603

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