The authors are grateful to Dr. Hans Hitzler (Division of Hematology/Oncology, The Hospital for Sick Children,
Toronto, Canada); Dr. Eyal Grunebaum (Division of Immunology, The Hospital for Sick Children, Toronto, Canada); Dr. Greg Denomme (Canadian Blood Services, Toronto, Canada) for their useful contributions to the chapter and to Dr Alvin Zipursky for his critical review and comments during the preparation of this manuscript. This chapter was prepared with the editorial assistance of Lu Ann Brooker.