Blood fractions refer to minute quantities of blood components that are collected from large donor pools.
For example, Anti-D (Win-Rho, Rho-Gam) is administered to Rh-negative women during and after pregnancy to prevent hemolytic disease of the newborn in subsequent pregnancies. The amount administered is about 2.5 mL (½ teaspoon), but it takes thousands of donors who are isoimmune (Rh-negative and exposed to Rh-positive blood and have developed the Rh antibodies) to synthesize the blood product. It is okay for a JW woman to take Anti-D, or rather it is up to her (her husband's?) Conscience™.
Having a baby with hemolytic disease of the newborn is awful. Even though we can treat the baby with massive phototherapy from the instant it is delivered, it can take a long time for the baby to get better, and in the meantime, the baby is separated from its mother. Sometimes though the phototherapy treatment isn't enough and exchange transfusions are needed so the baby won't die of oxygen deprivation as his red blood cells die off faster than they can be made. The other potential outcome without treatment is severe brain damage from bilirubin (a by-product of red blood cell breakdown). Any parent who would knowingly risk their baby's life like this by not taking a relatively simple precaution deserves to have child protective services all over their stupid asses.
And yes, you are correct, a JW woman who has become isoimmunized could potentially help other women protect their children from this horrible fate by donating their precious antibodies, but the WTS forbids it.