Re: in surrogate mothers, won't the antigen of the ovum react with an antibody?

An excellent question, and one that has baffled scientists for years. 

Firstly in the assisted fertility treatments involved in surrogacy an 
embryo (not a ovum) would be introduced into the uterus of the surrogate 
mother. An embryo used in a surrogacy situation is no different to an 
embryo produced by natural fertilization. It has come from sperm and ovum. 
That is to say that it is genetically, and therefore immunologically and 
phenotypically, a different human being. The question is, therefore, why 
is ANY fetus not rejected, through antigen/antibody interaction?

Antigens first appear on the embryo shortly after implantation (about 8-10 
days after fertilization) and are therefore perfectly capable to elicit 
immune responses and be rejected. The complete answer still eludes science 
to this day. Here are a few possible explanations

1.	Fetal and maternal blood systems are separated by the placenta 
(immunologically this is fetal tissue and therefore foreign). At the 
border between the mother and fetus the placental cells (trophoblasts) do 
not have “normal” antigens on their surface but special ones that might 
interact differently – this is still being researched.
2.	A special population of natural killer cells that interact with 
these antigens and regulate the maternal (mothers) immunology by cell 
signaling.
3.	The immunology of the mother is dampened possibly by the high 
levels of pregnancy hormones. 
4.	The separated system barriers also act like a filter and only some 
maternal antibodies (IgG’s) are allowed to pass through. It is believed 
that the fetus “mops up” these few antibodies before they can build up to 
cause problems. 

An exception to this is the rhesus factor of red blood cells, if the 
mother is rhesus –ve and the fetus is rhesus +ve immunological rejection 
can and does occur!