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.