Natalizumab (TYSABRI) is a protein-based drug that is manufactured by Biogen Idec in partnership with Elan Pharmaceuticals. Natalizumab is approved in the US and Europe for the treatment of Multiple Sclerosis (MS). The purpose of this study is to determine whether the amount of natalizumab (TYSABRI) that is present in your blood (plasma) can be reduced or eliminated by separating and removing the plasma and replacing it with other fluids, a process called plasma exchange.

Inclusion Criteria:

• considered by the Investigator to be free of signs and symptoms suggestive of any serious opportunistic infection
• willing to discontinue and remain free from concomitant immunosuppressive or immunomodulatory treatment (including interferon beta and glatiramer acetate) for the duration of the study
• willing and able to comply with the site's plasma exchange protocol which may require hospitalization or daily visits

Exclusion Criteria:

• considered by the Investigator to be immunocompromised
• history of, or available abnormal laboratory results indicative of any major disease that would preclude the administration of a recombinant humanized antibody immunomodulating agent for the duration of the study.
• condition(s) considered to be contraindication(s) for plasma exchange, including but not limited to bleeding diathesis, hypotension, or vascular access limitations