The Rh factor

Blood group antigens and related bacterial epitopes are polysaccharides. Such polysaccharide antigens induce immune responses in which no memory cells are generated. As a result, anti-blood group antibodies are always IgM (generated from primary responses) rather than IgG (generated from secondary responses). This is fortunate in pregnancy because IgM does not cross the placenta, so no harm comes to a fetus with a blood type that is not compatible with its mother’s. However, another red blood cell antigen, the Rh factor, can cause trouble for a fetus. As a protein antigen, the Rh factor induces immune responses in which memory cells are generated. Later exposure of these memory cells to the Rh factor leads to production of anti-Rh antibodies that are IgG.

A potentially dangerous situation can arise when a mother who is Rh-negative (lacks the Rh factor) carries a fetus that is Rh-positive, having inherited the factor from its father. If small amounts of fetal blood cross the placenta, which may happen late in pregnancy or during delivery, the mother mounts a humoral response against the Rh factor. The danger occurs in subsequent pregnancies with an Rh-positive fetus, when the mother’s Rh-specific memory B cells are exposed to the Rh factor from the fetus. These B cells produce anti-Rh IgG antibodies, which can cross the placenta and destroy the red blood cells of the fetus. To prevent this, the mother is injected with anti-Rh antibodies around the seventh month of pregnancy and again just after delivering an Rh-positive baby. She is, in effect passively immunized (artificially) to eliminate any fetal Rh-bearing red blood cells that cross the placenta before her own immune system responds to them and generates immunological memory that would endanger future Rh-positive babies.

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