Through Jacques Miller’s investigations of the influence of the thymus in leukaemia, the profound effects of thymectomy on lymphopoiesis and immunogenesis became evident. He established that the thymus was essential during early life for the complete development of a normal immune system. He also established that the thymus, even in adult life, is essential to maintain an adequate pool of immunologically competent cells. Grafting thymus tissue or injecting competent lymphocytes reversed the effects of thymectomy. Jacques showed that the thymus exerts its effects mostly by providing lymphocytes to the circulation. Antibody-forming B lymphocytes came from bone marrow.

He also showed that removal of the thymus at birth favoured the development of certain types of cancer induced by viruses and chemicals. This provided experimental evidence for the notion that normal immune reactions are necessary to limit the appearance of tumours. These and related discoveries have opened up wide areas of research, not only in the field of tissue transplantation but also in the management of certain immunological deficiency syndromes, allergic conditions and cancer.