Natalizumab, a recombinant and humanized IgG4 monoclonal antibody, binds to _4_1-integrin and blocks its interaction with vascular cell adhesion molecule-1 (VCAM-1). It is used to treat relapsing remitting multiple sclerosis and Crohn's disease. It is also the first targeted therapy to block an important mechanism for lymphocyte entry to the CNS and thus prevent acute demyelinating relapses.