These people are said to be sensitized. When exposed again to the antigen, the body remembers it and begins to produce antibodies to it which agglutinate the erythrocytes. This is particularly dangerous in the case of a pregnant female who is Rh negative and her second Rh positive child IF the first child sensitized her. The antibodies cross the placenta and destroy the infant's RBCs causing it be severely anemic. This tragedy need not occur nowadays due to the presence of Rho Gam which must be given within 48 hours or so after birth, miscarriage or abortion and provides passive immunity.
Passive immunity implies the mother is doing nothing which is true.