Re: How is white blood cell structure affected by intermarriage?

Your question is very interesting.  First, I'll explain a bit about why bone 
marrow donors must be matched with the recipient.  After that, I'll try to 
explain the genetics of how the system works.

Bone marrow donors must be matched with the recipients for two reasons.  
First, the recipient could reject the bone marrow, thus making the 
transplant not work.  This isn't that common, as the recipient is treated 
with drugs and radiation that knock out his immune system so that he can 
accept the transplant.

Second, the bone marrow after the transplant will begin producing white 
blood cells that are identical to the donor's.  If these cells recognize the 
recipients tissue's (skin, lung, kidney, liver, colon, etc.) as foreign 
(which will happen if there is not a good match) they will destroy the 
recipient's tissues, making him very sick.  Doctors call this graft-versus-
host disease.

The system that allows the white blood cells to recognize other tissues as 
foreign is called the HLA system.  Basically, six molecules on the surface 
of cells in your body can tell your own white blood cells that you are 
"self".  If your white blood cells see cells with any other form of these 
six molecules that don't match your own, they will see the cell as foreign, 
and destroy it.

The six surface molecules are called HLA-A, HLA-B, HLA-C, HLA-DR, HLA-DP, 
and HLA-DQ.  Each one of these is coded for by a separate gene, and in the 
population as a whole there are many versions of each molecule.  Within a 
single person, you have two versions of A, two of B, two of C, two of DR, 
two of DP, and two of DQ.  You got one set of A, B, C, DR, DP, and DQ from 
your mom, and one set from your dad.  That makes for a total of twelve 
molecules on the surface of your cells that could be recognized by someone 
else's white blood cells as foreign.  For the purposes of transplants, some 
of these are more important than others, especially A and B.

It turns out that people within a particular ethnic group tend to have some 
of the versions of these molecules in common.  Thus the likelihood of a 
match between donor and recipient usually turns out to be higher between two 
members of the same ethnic group.

When two people have children, whether they are of the same ethnic group or 
not, it is very unlikely that the two parents would have the same set of HLA 
molecules.  Their children will be a mixture of 6 molecules from mom, and 
six molecules from dad.  In this system there is no dominant or recessive, 
and all six from mom and all six from dad get expressed equally.

One instance where two people will be perfectly matched is the case of 
identical twins.  They have all 12 molecules matching, and so transplants of 
any type work especially well.

When you give blood to enter the bone marrow donor registry, they test your 
cells to see which HLA molecules you have.  If a person needs bone marrow 
and turns out to be a match with you, usually more tests of your blood will 
be done to confirm the match and make sure you are a suitable donor.

Hope this helps.

BRIAN EDELSON
edelsonb@medicine.wustl.edu