<qa>  What is the purpose of this study?The purpose of this study is to determine the safety, tolerability, and efficacy of natalizumab in individuals diagnosed with active Crohns Disease that are not in remission CDAI greater thanequal to 150 and are currently taking Remicade. It is thought that natalizumab may stop the movement of certain cells, known as white blood cells, into bowel tissue. These cells are thought to cause damage in the bowel leading to the symptoms of Crohns disease. 