Rh factor is a protein antigen.
A homozygous recessive individual is Rh-negative and lacks the Rh factor.
Incompatibility may occur if an Rh-negative mother carries a Rh-positive fetus.
She can develop antibodies
against the Rh factor if she receives some blood from the fetus during birth.
Subsequent pregnancies of Rh-positive fetuses can magnify this response,
and the antibodies can cross the placenta to attack the RBC of the fetus.