Hi debra,
Plasmapheresis or more exactly plasma exchange, is the exchange of the plasma, which is one of the blood constituants.
The effectiveness of that treatment is based on the fact that most of the antibodies are present in the plasma, so if you remove the plasma, you remove the antibodies.
That exchange is done by centrifugation of the blood which separates plasma from the other constituants. Based on the individual physical caracteristics, about a gallon of plasma is replaced.
Blood is usually extracted from one elbow vein and reinjected in the other arm after passing through the centrifuge and pumping systems.
One exchange last from 75 minutes to 2 hours depending on the vein capacity to sustain high blood flow rates and on the quantity of plasma to be exchanged.
When done in crisis conditions, one exchange is performed every 2 days and is repeated 4 or 5 times.
The effects are shown rather rapidly, a couple of days but unfortunately they depend on the quantity of antibodies which are hooked somewhere in the body and since they are not circulating, they cannot be removed.
I hope this helps...
Maurice.