|MadSci Network: Medicine| > Why don't blood group antibodies
cross the placenta and kill the baby. Hi - Antibodies against
ABO antigens are almost always IgM antibodies. The IgM antibody
consists of 5 antibody units connected together, in a star-like
pattern. The protein is simply too large to cross the placenta.
However, if a break in the placental-fetal barrier happens, the
mother may generate a smaller immunoglobulin, IgG, against the
ABO antigens on the fetal red cells. This Ig-type can cross the
placenta. If early enough in the pregnancy, she may develop
sufficient IgG antibody to affect the current pregnancy. The
presence of the antibody can certainly affect future ones. IgG
antibodies against the Rh(D) antigen were once tbe most common
cause of hemolytic disease of the newborn. In such cases the
mother lacks the Rh(D) antigen on her red cells but the baby
inherits the antigen from the father, and so has it on his or
her red cells. The mother may generate antibody when exposed to
the antigen from a pri