John W. Semple is a Canadian Scientist formally at St. Michael's Hospital and a Professor of Pharmacology at the University of Toronto.
He is currently a Professor of Transfusion Medicine at Lund University in Sweden.
In 1991, Semple, along with John Freedman, discovered a T helper cell defect in patients with the bleeding disorder called immune thrombocytopenia (ITP).
Very low platelet counts can lead to a bleeding diathesis and purpura.
Subsequently, this cytokine abnormality was shown by others to be responsible for many of the autoimmune mechanisms causing the disorder.).