Just found this on the NMSS website:

Donating blood
I have a friend who has MS and is taking Avonex. He would like to donate blood at our quarterly blood drive. Can he do this? Are there any reasons why he couldn’t?
XXXXXXX, Rhode Island

Dr. Stephen Reingold, vice president for Research at the Society: Blood banks will often refuse to accept donations from people with MS because of a concern that there may be a transmissible agent—for instance, a virus—present in the blood stream of people with the disease. While there is no solid evidence for such an agent in the blood, this is a reasonable cautionary policy. People with MS who are interested in donating blood should contact their local blood donor programs to determine their eligibility.


I also saw mention of theories about white blood cells moving out into the organs, etc., etc. etc.. Way over my head. Esentially, we don't know what causes auto-immune disorders so you're suspect until proven otherwise.