Christopher earned a PhD in immunology at the Sloan-Kettering Division of Cornell University Graduate School of Medical Sciences in 1979. He became a member of the Basel Institute for Immunology in Switzerland in 1980 where he worked until joining the Ontario Cancer Institute (OCI) as a senior scientist in 1987. In 1990, Christopher became the founding director of the Arthritis and Autoimmunity Research Centre as well as director of research at The Wellesley Hospital. In 1997, he returned to OCI, now part of the University Health Network (UHN), to assume the role of vice president, research at UHN. Christopher now serves as senior scientist at UHN and is a professor in the Departments of Medical Biophysics and Immunology at the University of Toronto.