What is the immunological problem?

Scientists have long had to grapple with the fact that is less surprising that some pregnant women reject their fetuses than that any fetus, in fact, makes it through to full-term birth. Why? Because the processes of conception, fertilization, and embryological development go against one of the basic tenets of nature: the body, as transplant surgeons well know, rejects anything is dose not recognize as its own. The embryo is only 50 percent its mother’s tissue; the other half, which comes from the father, is considered by her body to be foreign tissue.

In the very early days following conception, the trophoblast, the bundle of fetal cells of the developing embryo and placenta, actually comes in contact with the mother’s tissue and her blood as it attaches to the uterine wall. The mother’s blood cells make antibodies to this partly “foreign” tissue, as would be expected.

Normally, however, a pregnant woman will also make special antibodies that mask and protect these trophoblast cells from the antibodies formed. These special antibodies, peculiar to the pregnant state, are known as blocking antibodies.

A woman who becomes pregnant has naturally been exposed to her partner’s foreign cells through intercourse. Semen carries some tissue proteins, called antigens (that is, proteins that trigger an antibody response). It dose not matter how many intercourse you have had previously; even once is enough to exposed you to antigens that help produce the protective blocking antibody. But some women’s bodies just do not recognize their partner’s tissue as foreign enough.

Doctors used to think that in a curious reversal of normal situations, it is the underactive immune system not recognizing this tissue as foreign that prevents the “blocking antibody” from beingmade, not, as you might imagine, a widely aggressive immune system attacking all foreign tissue at will. It is now believed that certain men and women are genetically too similar. In this case, the woman adopts the husband’s antigens as her own.