In America, Alexis Carrel was the first to experiment in early transplantation by the transference of organs in animals. Carrel together with Emerick Ullmann performed "en masse" transplantation by removing both kidneys, aorta, and vena cava, taking the ureters with the kidneys for use in anastomosis.
Between 1900 and 1930, in this country and abroad, workers like Carrel and Ullmann were performing this type of experiment. Kidneys were most often used because of their simple vascular supply. The ureters functioned within minutes and gave an index to work with. Although Carrel recognized the transferred kidneys functioned normally, put out urine and maintained life, he was unaware what was causing their loss. The concept of rejection was not yet understood.
The realization of rejection taking place was thought to be due to a process quite distinct from infarction (ie., loss of arterial blood supply) infection, or inflammation. Initially, the term rejection indicated a process by which the host was not accepting of the new organ. It was not long before it was understood that there might be some relationship involving the immunologic process by which an organism combats invading bacterial infection.