Natalizumab is primarily used in the treatment of multiple sclerosis (MS) and Crohn's disease. In MS, it works by preventing immune cells from crossing the blood-brain barrier, thereby reducing inflammation and nerve damage. This helps in decreasing the frequency of relapses and slowing disease progression. For Crohn's disease, natalizumab is used in cases where other treatments have failed. It inhibits the migration of immune cells into the intestines, reducing inflammation and helping to manage symptoms. Due to its mechanism of action, natalizumab is particularly effective for patients with relapsing forms of MS and moderate to severe Crohn's disease who have not responded well to conventional therapies. However, its use requires careful monitoring due to the risk of serious side effects, such as progressive multifocal leukoencephalopathy (PML).