Immunization against the Anticoagulating Effect of Hirudin

In former experiments we found that various substances, among them hirudin, injected intravenously into mice inhibited the growth of mouse tumors.’ We found, furthermore, that in the course of the injections of the solutions of colIoida1 copper and of hirudin, immunity was produced against the effect of these substances, the rate of growth of the tumors serving as an indicator of the degree of immunity reached. We were able to show that this immunity was of a twofold origin; namely, it had its source, first, in a process of immunization taking place somewhere in the organism of the injected animal; and, second, in changes taking place in Dhe tumor cells themselves as the result of the injections, the tumor cells themselves becoming immune against the action of these substances. This latter kind of immunity could be transmitted through several generations of tumor cells. In order to obtain a further insight into the first component of this process of immunization, we decided to study the immunity produced through injection of hirudin into mice, using the effect of repeated injections on the production of substances affecting the coagulation of the blood as an indicator. It has been found in former experiments by Ledoux2 that a few injections of hirudin into rabbits do not produce immunity. Wendelstadt? believed that he produced an antibody against hirudin. After numerous int,raperitoneal or subcutaneous injections of hirudin into rabbits, he obtained an antibody which inhibited the