Tysabri is not chemotherapy, it is a targeted treatment that works by blocking the migration of lymphocytes (a type of immune cell) from the lymph nodes, across the epithelium, and into inflamed tissue. It binds to a receptor that is present on all leukocytes apart from neutrophils and prevents them from binding to their counter receptors. It may also act in several other ways to prevent the further recruitment and inflammatory activity of activated immune cells. Tysabri belongs to the class of medicines known as integrin receptor antagonists. It may also be called a selective immunosuppressant or a monoclonal antibody.

The way Tysabri works specifically in MS or Crohn's disease is not completely understood. In MS it may be due to a reduction of lymphocyte migration into brain tissue and a reduction in plaque formation. In Crohn's disease, Tysabri blocks the interaction of a particular protein called α4β7 integrin with the endothelial receptor MAdCAM-1. MAdCAM-1 is mainly expressed in gut tissue, and expression of MAdCAM-1 is increased at active sites of inflammation in people with Crohn's disease, and may significantly increase the recruitment of leukocytes to the mucosa.

Tysabri (natalizumab) may be used to treat adults with multiple sclerosis or Crohn's disease and is given once every four weeks by a registered infusion center or pharmacy that is enrolled in the Touch Prescribing program. A headache, fatigue, joint pain, and infections are the most common side effects reported although rarely, it may cause more serious side effects such as liver failure, severe herpes infections, or PML. It may not be suitable for people who already have a weakened immune system.