[medical citation needed] Natalizumab, is a monoclonal antibody which targets a protein called α4β1 integrin on white blood cells involved in inflammation.

[10] By attaching to integrin, natalizumab is thought to stop white blood cells from entering the brain and spinal cord tissue, thereby reducing inflammation and the resulting nerve damage.

[10] The most common side effects are urinary tract infection, nasopharyngitis (inflammation of the nose and throat), headache, dizziness, nausea, joint pain and tiredness.

It was subsequently withdrawn from the market by its manufacturer after it was linked with three cases of the rare neurological condition progressive multifocal leukoencephalopathy (PML) when administered in combination with interferon beta-1a, another immunosuppressive drug often used in the treatment of multiple sclerosis.

However, twenty-four cases of PML had been reported since its reintroduction by October 2009, showing a sharp rise in the number of fatalities and prompting a review of the chemical for human use by the European Medicines Agency.

[14] In the European Union, it has been approved only for multiple sclerosis and only by itself as the initial cases of PML, and later the fatalities, were said by the manufacturers to be linked to the use of previous medicines by the person.

[1][9] It is indicated to treat clinically isolated syndrome – a single, first occurrence of multiple sclerosis symptoms; relapsing-remitting disease – a type of multiple sclerosis that occurs when people have episodes of new neurological symptoms followed by periods of stability; and active secondary progressive disease – when, following a relapsing-remitting course, patients experience gradual disability worsening with continued relapses.

[20] In the European Union, natalizumab is indicated as single disease modifying therapy in adults with highly active relapsing remitting multiple sclerosis for the following patient groups: The US prescribing information for natalizumab contains a boxed warning about the increased risk of progressive multifocal leukoencephalopathy,[16] a viral infection of the brain that usually leads to death or severe disability.

[26] In 2016, EMA recommended all people taking natalizumab should undergo full MRI scans at least once a year due to concerns of progressive multifocal leukoencephalopathy (PML).

[32] Common adverse effects include fatigue and allergic reactions with a low risk of anaphylaxis,[33] headache, nausea, colds and exacerbation of Crohn's disease in a minority of patients with the condition.

[35] The interaction of the α4β7 integrin and the addressin (also known as MADCAM1) endothelial cell receptor is believed to contribute to the chronic bowel inflammation that causes Crohn's disease.

Addressin is primarily expressed in the endothelium of venules in the small intestine and are critical in guiding T-lymphocytes to lymphatic tissues in Peyer's patches.

Animal models have found higher levels of VCAM-1 expression in mice with irritable bowel syndrome and the VCAM-1 gene may also play a part in CD but its role is not yet clear.

[9] Natalizumab appears to interact with other immune-modulating drugs to increase the risk of progressive multifocal leukoencephalopathy (PML), an often-fatal opportunistic infection caused by the JC virus.

[9] By January 2010, the United States Food and Drug Administration reported a total of 31 confirmed cases of PML associated with natalizumab.

[43] Natalizumab was originally approved for treatment of multiple sclerosis in 2004, through the FDA's accelerated Fast Track program, due to the drug's efficacy in one-year clinical trials.