MS attacks the brain, spinal cord and optic nerves of MS patients, causing pain, fatigue, vision and coordination difficulties. Cross transformed the field of MS research when, by studying an animal model of MS, she discovered that immune cells known as B cells play a critical role. The findings, which led her to study B cells in people with MS, were a direct challenge to the accepted dogma of the time, which held that MS was caused by a different immune cell type, the T cell. Cross later led the first clinical trial of rituximab, a B cell-depleting therapy for MS, which showed an 88 percent reduction in new MS lesions. While rituximab was never approved to treat MS, it helped pave the way for a related compound, ocrelizumab, which was approved as a therapy for MS by the Food and Drug Administration in 2017.