Incompatibility of mother and fetus with respect to many antigens is a constant feature of pregnancy as the fetus becomes a homeotransplant in relation to the maternal body having inherited 50% of its genes from the father. Nature has foreseen many mechanisms that prevent this incompatibility from fulfilling itself. Nevertheless, clinical practice has seen cases when the defense mechanism fails to work (early toxicosis, antiphospholipid syndrome, hemolytic disease of newborn, and other).
15.1. DEFINITION
Allo- or isoimmunization is production of antibodies by the immune system in response to foreign protein antigens of another (Latin Allos - other, foreign) individual of the same (Latin Isos - same) species. This commonly occurs in the recipient's body in response to other person's erythrocytes entering his blood carrying on their surface antigens that the recipient does not have. This can occur in case of
• transfusion of whole blood or erythromass;
• sharing one syringe by two drug addicts;
• pregnancy.
A certain amount of fetal erythrocytes commonly enter maternal blood during the third stage of labor when the placental barrier has been disturbed (detachment of placenta), and fetal erythrocytes pass to maternal circulation through gaping vessels in the placental bed, that is, feto-maternal transfusion actually happens. It was established that the rate and extent of bleeding grows with gestational age. The rate of feta-maternal transfusion amounts to 7, 16 and 29% in the first, second and third trimesters, correspondingly.
15.2. EPIDEMIOLOGY
15.2.1. Prevalence (U.S. data)
Until modern approaches were adopted, 1% of all pregnant women developed Rh-alloimmunization. As a consequence of routine anti-Rh-prevention the incidence rate of Rh-sensitization dropped from 45 cases per 10 000 deliveries (4.5%) to 10.2 cases per 10 000 deliveries (1%).