Among the attendees at the seventh International Congress of Hygiene and Demography in London in August 1891 there were two ambitious doctors, Emil Behring (1854–1917), an assistant of Robert Koch at the newly founded Institute for Infectious Diseases in Berlin, and Émile Roux (1853–1933), head of the department for technical microbe research at the Pasteur Institute in Paris. In the years that followed, both would become key protagonists in the experimental production of the diphtheria serum, which ultimately led to the mass production of serum for human use starting in 1894. Their lectures at the London congress were concerned with the findings that Behring had published in the winter of 1890: Together with Shibasaburo Kitasato (1852–1931) he had demonstrated that blood serum from guinea pigs immunized against tetanus could be used to immunize and even to cure other animals (Behring and Kitasato Reference Behring and Kitasato1890). Behring had also immunized guinea pigs successfully against diphtheria (Behring Reference Behring1890). By the summer of 1891, he had also proved the therapeutic effect of this serum (Behring Reference Behring1891, 1395). Behring explained that the serum of immunized animals acted by neutralizing the toxin produced by the microbes, thus working by an “antitoxic” effect as he termed it, and he soon became famous (Behring and Kitasato Reference Behring and Kitasato1890, 1114 n. 1). His finding was based on the experimental isolation of the diphtheria toxin by Roux and his assistant Alexandre Yersin (1863–1943) in 1888 – and Behring did not hesitate to acknowledge publicly in London the importance of their “classic work” and the practical impact that it had had on his own research (Behring Reference Behring1891, 1395).

Likewise, Roux praised the experiments conducted by Behring and Kitasato in his own lecture, but conceded only that the blood serum of immunized animals had a “toxinicidal” power (pouvoir toxinicide), whereas Behring had described it as having a direct “immunizing” or “healing” potency that could be transferred to other animals. Roux insisted that the antitoxic effect should not be equated with immunity. In Roux's view, immunity was induced primarily by “phagocytosis,” that is to say as part of the “struggle between microbes and cells” (Roux Reference Roux1891, 527–529).Footnote ¹ Behring, on the other hand, was convinced that both the “acquired” and “conferred” forms of immunization were independent of the blood cells and should be attributed to the antitoxic power of the serum. In this context, Behring spoke of “cell-free blood.” Thus, in front of the international audience he proclaimed: “Like the vital force in earlier times, so today the mysterious forces of the vital cell play a paralyzing role in our theories of immunity” (Behring Reference Behring1891, 1397). Roux, for his part, was equally categorical in his defense of phagocytosis: “Far from being unsettled by opposing theories, it capitalizes upon them; that, gentlemen, is a guarantee of its solidity, is it not?” (Roux Reference Roux1891, 533). The theory of phagocytosis had been developed by the Russian biologist Elie Metschnikoff (1845–1916), who was not only head of the department for morphological microbe research at the Pasteur Institute, but also a close friend of Roux. At the congress in London, Metschnikoff defended his theory in a comment from the floor, arguing that the “army of phagocytic cells is really a salutary force formed in the struggle for the existence of the animal organism” (Metschnikoff Reference Metschnikoff and Shelly1892, 183). When it came to Behring's experimental findings and his immunity-theory, Metschnikoff stated that he accepted the significance of the antitoxic effect of sera, but only with respect to the conditions of the “special immunity” against tetanus and diphtheria toxin (ibid.).

As I will show in this article, the disputes between Behring and Roux as well as between Behring and Metschnikoff can be understood in terms of Andrew Mendelsohn's illuminating thesis about the “two cultures of bacteriology,” which focuses on the distinction between the research schools of Robert Koch, founder of the Institute for Hygiene in 1885 and from 1891 head of the Institute for Infectious Diseases in Berlin, and that of Louis Pasteur, founder of the Pasteur Institute in Paris in 1888. Mendelsohn has argued that because they inhabited different cultural worlds of experience, Koch, Pasteur, and their respective disciples developed profound differences in their understanding of microorganisms and disease – with Koch's school regarding bacteria as the specific, unidirectional, determinant cause of disease, while the Pasteurians stressed an interactive and ecological model of disease: They saw disease as evidence of a “struggle for life” (that is, for oxygen and nutrients) between microbes and cells influenced by variable milieu factors – an account that was obviously developed by Pasteur without reference to the Darwinian concept of “struggle” (Mendelsohn Reference Mendelsohn1996, esp. chap. 2). While Mendelsohn's main thesis emphasizes the profound differences between the two research schools, he also shows the (often tacit) adoption of experimental methods, scientific concepts, and institutional structures associated with Koch's school by the Pasteurians and – to a lesser extent – vice versa in the late 1880s and subsequent years (ibid., esp. chap. 4). Thus, Mendelsohn also diverges from the commonly held view that nationalistic rivalries deriving from the Franco-Prussian War resonated in the scientific debates between Pasteur and Koch and their disciples to such an extent that there was virtually no productive exchange (Silverstein Reference Silverstein1989, chap. 3). Mendelsohn does not develop his thesis in relation to the diphtheria serum research in the first half of the 1890s – research that was accompanied by acrimonious disputes over theories of immunity. In looking closer at this time period, I will enhance the complex picture of the “two cultures of bacteriology” by showing how differences between the two research schools were reconfigured through a reciprocal demarcation of scientific concepts in the direct and indirect exchanges between Behring, on one side, and Roux and Metschnikoff on the other. This was in part acted out as a public academic quarrel. And while I will demonstrate how, on both sides of the border, the peculiarities contributed to divergent steps in the experimental development of the diphtheria serum (with its various dead ends as well as its consolidating processes), it will also become clear that the private exchanges between researchers in Berlin and Paris in the early 1890s were nevertheless respectful and open minded enough to allow for a mutual adoption of experimental practices. To characterize these exchanges at this time as “cooperation” (Weindling Reference Weindling and Pickstone1992, 75) would be, in my opinion, off the mark given the ostentatious demarcations that the protagonists engaged in.Footnote ²

This perspective is especially helpful in studying serum research at the Pasteur Institute because the historical sources for the period from 1890 to 1894, when the serum was transformed into a stable and standardized therapeutic agent, are very fragmentary compared to the German case. It suggests that the “gaps” and “lacunae” in the (documented) research process – compared to the German situation – might in part be due to a divergence in the style of reasoning and experimenting at the Pasteur Institute. A closer analysis of the French serum research remains a desideratum and although the German side of the story has been told more than once,Footnote ³ there are features missing that are of key importance for the rapid transformation of the erratic experimental extraction of the serum into a stable production process.

The paper will proceed as follows: First, I outline the practical implications of the different research cultures for the early experiments on diphtheria immunization around 1890. Those experiments addressed the issue of “weakening” or “attenuating” the infective agent. Second, I will reconstruct the experimental phase in the development of an effective immunization technique that enhanced the therapeutic value of the diphtheria serum and enabled large scale serum production simultaneously in Berlin and Paris in 1894. Here, I will assess how different concepts of immunity were simultaneously specified in a tense interplay between Behring, Roux, and Metschnikoff that did not inhibit the mutual adoption of experimental practices. In the third and final part of this article, I will show how divergent concepts of immunity led to differences in the use of methods for evaluating the “efficacy” of the therapeutic serum.

“Attenuation” or the “Weakening” of a Pathological Agent

What proved to be a productive point of departure for Behring's research into diphtheria immunization, set Roux off on a quest that turned out to be a dead end. The beginning of immunization research was marked by an experimental exploration of methods designed to reduce the pathogenic effect of the diphtheria bacteria or their purified toxin, so that test animals could recover and thereby acquire immunity. Behring's approach involved looking for chemical means to achieve this goal, which he – like other German researchers – described as Abschwächung (weakening) (Behring Reference Behring1891, 1394). Here Behring benefited from his experience with antiseptic chemicals and his wide-ranging knowledge of their properties, reactions, and clinical effects. Behring had worked intensively on this subject in the 1880s, turning himself into an expert on the question of disinfectants, a subject of great importance in military medicine. As Jonathan Simon has argued, this practical achievement in working with antiseptics served as the basis for Behring's initial approach to the therapy of infectious diseases. That approach involved a kind of “inner disinfection” using antagonistic substances to render microbes innocuous within a living body (Behring and Kitasato Reference Behring and Kitasato1890, 1114 n. 1; Simon 2006). In pursuing this line of reasoning, Behring found that the inorganic antiseptic iodine trichloride could be used to heal guinea pigs with diphtheria. Nevertheless, a direct trial of iodine trichloride on infected children failed because of the chemical's unacceptable caustic effects (Behring Reference Behring1890, 1146). In 1889–90, the search for a therapeutically effective and safe minimal dose of an antiseptic that could be administered to animals after infection with diphtheria was redirected toward experiments aimed at diminishing the toxic effect of diphtheria (and other infectious diseases) in order to induce immunity by chemical or mechanical means (Behring Reference Behring1890; Behring Reference Behring1891, 1395).Footnote ⁴ These experiments targeted either the infective agent or the test-animals and corresponded with other researchers' claims about the sera of immunized animals and its ability to neutralize specific bacterial infections (Zeiss and Bieling Reference Zeiss and Bieling1941, 55–57). In the winter of 1890, Behring and his collaborators had achieved convincing results in their immunization of guinea pigs against diphtheria and had shown that it was possible to confer the immunizing power of their blood sera on other animals. This led to Behring's and Kitasato's famous publication of 1890.

In contrast to Behring's progress, Roux and Yersin attempted to manipulate the pathogenic power of diphtheria cultures or the filtered toxin using heat, antiseptic chemicals, and other methods in such a way that immunization would result.Footnote ⁵ These efforts were undertaken between 1888 and 1890 and failed to produce consistent results. As Mendelsohn has argued, this period of their work was marked by a return to Pasteurian research axioms, with the researchers trying to vary the “virulence” of the diphtheria cultures – conceived as a kind of biological property – in order to find a stable attenuated microorganism that could be used to develop a method for “microbic vaccination”Footnote ⁶ (Mendelsohn Reference Mendelsohn1996, chap. 5).Footnote ⁷ One approach was to try to simulate the clinical conditions of patients who had recovered spontaneously from diphtheria in in vitro cultures. They also searched for “naturally attenuated” cultures and interpreted pseudo-diphtheria-bacilli as such rather than as a separate species of bacteria (as was commonly believed at the time) (Roux and Yersin Reference Roux and Yersin1890, 418 and 421). In continuing along this line of reasoning, they not only failed to induce immunity with their (supposedly) “attenuated” cultures; they were also led even further away from developing practical techniques for immunization. In their publications they showed themselves increasingly interested in a biological “hereditary attenuation” of microbes that they distinguished from a chemical or mechanical deactivation or weakening that would not affect subsequent generations (ibid., 415). At the congress in London in the summer of 1891, this distinction was again outlined in the introductory remarks of Roux's speech. But there were no indications in his lecture (or in other sources) that, in the meantime, he or his collaborators had been conducting systematic immunization experiments against diphtheria since Yersin had left for Indochina at the end of the summer of 1890.Footnote ⁸ Thus, it seems likely that the different orientations of Behring and Roux in their respective research cultures resulted in Behring making a productive start on diphtheria serum research, while Roux found himself diverted from this subject.

Immunization and Immunity

Given Roux's conceptual and practical distance from diphtheria immunization, the question arises: How did he return to diphtheria serum research? Roux later declared that he returned to diphtheria only after conducting a large number of experiments on immunity against tetanus (Roux and Martin Reference Roux and Martin1894, 614). According to him, he began with tetanus because, compared with diphtheria, attaining an antitoxic serum against it was easier (ibid., 609). This had also been the experience of Behring, whose tetanus experiments, conducted more or less parallel to the diphtheria experiments, were initially more promising. In the words of Louis Martin (1864–1946), who collaborated with Roux on diphtheria after Yersin had left, the underlying difficulty was due to the complexity of achieving a diphtheria toxin of constant strength (Sevestre and Martin Reference Sevestre, Martin, Grancher, Comby and Marfan1896).Footnote ⁹ A further hint that this was indeed an initial problem can be found in notes that Roux took on Behring's method of preparation and preservation of diphtheria toxin, published in March of 1892 together with his collaborator Erich Wernicke (1859–1928). The preservation method with carbolic acid, which would allow keeping the force of the toxin nine months and even longer (Behring and Wernicke Reference Behring and Wernicke1892, 14), was probably of especially great interest for Roux because he and Yersin – at least in 1889 – had only been able to conserve their toxin for five months (Roux and Yersin Reference Roux and Yersin1889, 281–282). A later review of the diphtheria serum experiments shows that the Pasteurians did not adopt Behring's conservation method, but Roux's notes indicate his concern about which was the best method for conservation.Footnote ¹⁰

In any case, according to Roux and Martin the team at the Pasteur Institute started their road back to diphtheria by dealing first with tetanus immunization. Around the summer of 1891, Roux teamed up with Louis Vaillard, a professor at the military medical school at Val-de-Grâce, who, in the first half of 1891, had already replicated Behring's method of diminishing the strength of tetanus toxin using (among other methods) iodine trichloride as a way of achieving immunization (Roux Reference Roux1891, 527). Based on their success in animal experiments, they tested their tetanus serum on humans in the summer of 1892, but with disappointing results.Footnote ¹¹ Meanwhile, Roux and his collaborators gradually returned to the subject of diphtheria immunization.Footnote ¹² Whereas it seems that such experiments – at least those on large animals – were started mainly in 1893, a preliminary diphtheria immunization experiment using a horse had already been performed on the third of April 1892 using virulent diphtheria cultures that were administered first in small, then successively augmented dosages (Roux and Martin Reference Roux and Martin1894, 623). This was done at nearly the same time as the first tetanus immunization of a horse, which began on the fifth of April – with the two horses being of the same race, color, age, and weight (Roux and Vaillard Reference Roux and Vaillard1893, 133).Footnote ¹³ Their second documented case of equine diphtheria immunization – in which they used a mixture of diphtheria toxin with a gram solution (containing iodine) – took place in April 1893 (Roux and Martin Reference Roux and Martin1894, 636). While in the case of tetanus immunization of horses they explicitly followed the method developed by Behring involving a mixture of the toxin and iodine trichloride (Roux and Vaillard Reference Roux and Vaillard1893, 77 and 133), in equine diphtheria immunization they had no direct model to follow.

Shortly before the first horse experiment, Roux had received via Metschnikoff a recent article by Behring and his colleague Wernicke.Footnote ¹⁴ In this publication for the Zeitschrift für Hygiene from March 1892, Behring and Wernicke presented a diphtheria serum derived from sheep that was already highly effective in animal experiments. But they also clarified that, given their limited private funds, they were unable to produce serum in sufficient quantities for well-founded clinical tests on humans (Behring and Wernicke Reference Behring and Wernicke1892). The article made it clear not only to German pharmaceutical companies and the Prussian Ministry of Culture how promising this research into diphtheria serum had become,Footnote ¹⁵ but also made the message clear to Roux: he noted the main arguments of the article, the immunization and evaluation methods it introduced, and some of the experimental results.Footnote ¹⁶ The protocols of the ensuing experiments on diphtheria immunization by Roux and his collaborators, first published in 1894, show that, on the whole, they adopted the methods described by Behring and Wernicke (Roux and Martin Reference Roux and Martin1894). It appears, however, that the first French equine experiment of April 1892 using virulent diphtheria cultures had not yet been tried in this form by the Germans. Nevertheless, it seems probable that the German article encouraged the French researchers to undertake systematic diphtheria immunization experiments again, beginning immediately with large animals. Hence, there is no evidence to support an assumption that Roux did not pursue diphtheria serum experiments and that he subsequently adopted the immunization procedures on horses that were worked out by Behring with the help of Paul Ehrlich (1854–1915) in Berlin in the second half of 1893.

In Paris, the immunization-experiments with large animals were carried out by Edmond Nocard (1850–1903) at the veterinary school at Maison Alfort. The availability of horses for experiments thanks to the collaboration with the veterinary school provided Roux and his colleagues with a practical advantage over Behring and Wernicke at this stage of diphtheria serum research. Besides, in 1892 at least, Behring was convinced that sheep, not horses, were the most “efficient” producers of a therapeutic diphtheria serum (Behring and Wernicke Reference Behring and Wernicke1892, 11). It seems that Behring believed horses to be poor serum producers, because they were not highly sensitive to inoculation with diphtheria toxin; at least that's what Roux suggested about Behring's motives (Roux and Martin Reference Roux and Martin1894, 614).Footnote ¹⁷ One has also to consider that, initially, Behring and Wernicke themselves had financed the purchase and upkeep of large animals for their diphtheria serum research (unlike the tetanus experiments that were financed by the Ministry of Agriculture and carried out in cooperation with the veterinary college).Footnote ¹⁸ The lack of financial support for their research from the university or directly from the state, despite the international recognition of Behring's findings (Lister Reference Lister and Shelly1892), can partly be ascribed to the failure of Koch's tuberculin treatment for tuberculosis, which had spawned skepticism about the potential success of bacteriological therapies (Wernicke Reference Wernicke1931, 161).Footnote ¹⁹ Furthermore, Behring was himself ambivalent about state funding: From the outset of diphtheria serum research, he was interested in the prospect of making money from a therapeutically useful serum. This, however, required that the research not be conducted at the state-funded Institute for Infectious Diseases. Behring – like all the other employees of the institute – had signed a declaration waiving all claims to financial benefits derived from his research at the institute.Footnote ²⁰ The rule was enforced by the Prussian Ministry of Culture in the spring of 1891, after Koch's position had been weakened by the looming therapeutic failure of his tuberculin.Footnote ²¹ With an eye toward circumventing these institutional restrictions, Behring persuaded Koch to terminate the diphtheria serum project at the Institute for Infectious Diseases and continue the project using private funds, thereby retaining a right to profit from any future research results.Footnote ²²

This is the background to Behring's and Wernicke's article that indirectly sought large-scale funding for their promising diphtheria-serum research. In May 1892, Behring began discussions with the Farbwerke Hoechst and by July he had purchased some sheep and rented a stable in Berlin with company funds (Zeiss and Bieling Reference Zeiss and Bieling1941, 100). Having obtained funding from Farbwerke Hoechst, Behring negotiated with his superiors in the Prussian War Ministry and the Ministry of Culture, seeking state approval for the cooperation with Hoechst and for the right to make money from the venture should the serum prove to be successful.Footnote ²³ Behring also discussed another option that foresaw state funding of serum research, coupled with a university chair for himself.Footnote ²⁴ At the end of 1892, these negotiations still showed no signs of progress (and Behring did not get a chair until 1895). In this situation, he forced Althoff to accept his cooperation with Farbwerke Hoechst and signed a contract with the company on the twentieth of December 1892.Footnote ²⁵

Just at the time when Behring tried to come to terms with the Ministry of Culture and Farbwerke Hoechst, two other researchers claimed that they had attained diphtheria serum that was more potent than his. One of them was Wernicke and the other a private pediatrician named Hans Aronson (1865–1919), who would later cooperate with the pharmaceutical company Schering in diphtheria serum production. Especially in the winter 1892–93, Wernicke's work had the potential to compete with Behring's endeavor. By the summer of 1892 the direct collaboration between Behring and Wernicke had already ceased after a vehement quarrel that was probably as much personal as it was professional.Footnote ²⁶ On his own without Wernicke, Behring failed to generate any very potent serum (Schulte Reference Schulte2000, 77–78). Nevertheless, Behring was probably under pressure to produce results for Farbwerke Hoechst and so started clinical testing of sera in November 1892 in cooperation with Otto Heubner (1843–1926) from the Pediatric Clinic at the University of Leipzig.Footnote ²⁷ Alongside his low-potency-sera, Behring also sent some very high potency serum to Heubner. This serum had come from Wernicke; it was the product of his experiments on dogs that he had started of his own accord in the summer.Footnote ²⁸ Obviously, Wernicke seized the opportunity to have his dog-serum clinically tested and was again willing to cooperate in some limited fashion with Behring. On the nineteenth of December he presented the dog-serum to the Berlin scientific community (Wernicke Reference Wernicke1893a, 192). Just two days later Aronson, too, showed up with a dog-serum that he claimed to be much more potent than the latest published values for Behring's serum (Aronson Reference Aronson1893). At the beginning of February 1893, when Wernicke again presented his very potent dog-serum to the scientific community (Wernicke Reference Wernicke1893b), Behring was still trying to augment the efficacy of his sheep-serum.Footnote ²⁹ The dog-serum had delivered positive therapeutic results when administered to infected children in tests along with Behring's serum in the ward of the Institute for Infectious Diseases (Behring, Boer, and Kossel 1893, 393). These clinical trials revealed that Behring had finally caught up with the potency of Wernicke's serum. In light of the positive clinical evaluation (made possible with the help of Wernicke's dog-serum), Farbwerke Hoechst supplied Behring with additional money for his work.Footnote ³⁰ As Wernicke went on a journey to America from May until October 1893, a potential competitor departed the stage (Schulte Reference Schulte2000, 11). This, however, is only half of the story about how Behring succeeded in surpassing Wernicke and other German competitors – I will later return to the other half, which concerned the method for evaluating the efficacy of the serum.

Based on the experiments of his colleague Ehrlich at the Institute for Infectious Diseases in the summer of 1893, Behring adopted a method of increasing immunity through injections of pure diphtheria toxin of a constant high potency (Behring Reference Behring1893c, 281): In experiments on the prophylactic potential of milk taken from immunized dairy animals, Ehrlich had successfully tested this method, although he had used bacteria cultures instead of toxin to achieve immunity (Brieger and Ehrlich Reference Brieger and Ehrlich1893, 337; Ehrlich, Kossel, and Wassermann Reference Ehrlich, Kossel and von Wassermann1894, 353). In October, Behring drew up a contract with Ehrlich concerning the immunization of goats and a cow against diphtheria.Footnote ³¹ With the help of Ehrlich's technique Behring now turned to horses as Roux and Nocard already had: At the end of November 1893, he first announced the successful diphtheria-immunization of horses (Behring Reference Behring1893a, 1254; Behring Reference Behring1893c, 281).

At the beginning of 1894, both French and the German researchers possessed sufficient amounts of highly effective serum to perform large scale clinical tests on children (Chaillou and Martin Reference Chaillou and Martin1894; Ehrlich, Kossel, and Wassermann Reference Ehrlich, Kossel and von Wassermann1894). When Roux presented the successful research of the Institute Pasteur in public, he in fact alluded to the “antitoxic power” of the diphtheria serum, but declared at the same time his adherence to the theory of phagocytosis (Roux Reference Roux1894). He even explained the immunity conferred by serum as a form of active acquisition of immunity (acquise sur-le-champ) that worked by “stimulating” the “very vivacious cells” to exhibit a phagocytic reaction. He thus finally dismantled the antitoxin-concept and stood firmly on the side of phagocytosis-theory: “The preventive sera are stimulating and not antitoxic” (Roux and Martin Reference Roux and Martin1894, 624).Footnote ³² Nevertheless, in his course on microbiology in 1893, Roux respectfully acknowledged the antitoxin-theory as a contribution – in addition to phagocytosis – to the understanding of immunity, yet without supplying a coherent explanation binding the concepts together.Footnote ³³ To speak of antitoxins in his course was more likely due to a general acknowledgment of the experimental work of Behring and not (at least not until 1895) an effort at theoretical synthesis. Roux's recognition of Behring's work led to a situation in which priority quarrels were avoided: While he presented the clinical success of the French diphtheria serum at the Congress of Hygiene and Demography in Budapest in September 1894, Behring did not attend the congress. The French press thus took it for granted that Roux was the father of the therapeutic diphtheria serum. But Roux distanced himself from this assertion.Footnote ³⁴ At Roux's instigation, Behring was even named an officer of the French Legion of Honor in 1895 along with himself (Zeiss and Bieling Reference Zeiss and Bieling1941, 173).Footnote ³⁵

As I have argued, Roux's diphtheria serum experiments on larger animals were more or less modeled upon the experiences published in 1892 by Behring and Wernicke. Theoretical differences did not prevent his benefiting from practical experience gained in Berlin. The same holds true for Behring, as shown by his correspondence with Metschnikoff beginning in the autumn of 1891, thus right after the International Congress of Hygiene and Demography in London.Footnote ³⁶ From the outset and up until 1894, the correspondence revolved around differences in the explanation of immunity. The prelude to the dispute was Metschnikoff's offer to collaborate scientifically in November 1891. In his letter he wrote: “We can only support each other just like phagocytes and bacteria” (Metschnikoff to Behring, 29 November 1891, in AIP, Mtc 2, Doc. 27895). But Behring was eager to distance himself from the phagocytosis concept in order to enhance his own theory. In an article that appeared in March 1892 along with the one he had written with Wernicke in the Zeitschrift für Hygiene, Behring presented his “humoral” hypothesis, which emphasized the decisive and direct role played by the “cell-free blood” in immunization and healing. In this article, he claimed that, unlike the “cellular” hypothesis, his humoral one had been empirically confirmed (Behring Reference Behring1892, 1–2). Nevertheless, in the other article coauthored with Wernicke, he admitted that – unlike the immunity conferred by the serum – the successive active acquisition of immunity might result from a “ferment-like effect” of the “healing substance,” which therefore only provided the initial “stimulus for certain changes in the treated organism” that then produced the resulting immunity.Footnote ³⁷

Interestingly enough, Roux misinterpreted this passage in his personal summary of the article (as Roux had only a rudimentary knowledge of German, he probably had someone translate the article for him). In his notebook, he wrote: “They think the therapeutic serum acts like ferments that provoke changes in the organism from which immunity results” (“Cours de microbie technique,” November/December 1891, in AIP, Rou 7, Doc-No. 8863). Roux thus transformed assertions about actively acquired immunity against toxin into assertions about immunity induced by the serum. Metschnikoff was more aware of a potential misreading of this passage, as well as the potential of such a misreading to guide Behring towards adopting the cellular or phagocytosis theory. In his next letter to Behring, Metschnikoff cited the relevant passage and suggested that Behring had discreetly disavowed the antitoxin theory. At the same time, Metschnikoff mentioned that in the meantime he had become convinced that the antitoxins could not possibly make a substantial contribution to the phenomenon of immunization.Footnote ³⁸

Behring was quick to reject these propositions. In a letter to Metschnikoff at the end of March 1892, he declared that he was now in a position to identify the antitoxin as an excretion contained in urine. Moreover, his latest experiments on immunization against tetanus would soon prove that the acquisition of immunity against the toxin was also due to a chemical process induced by the poison's action on the cell-free blood serum.Footnote ³⁹ A publication of Behring's that appeared in 1893 again stressed that he generally considered the phagocytosis-theory to be a metaphysical obstacle in the path of therapy-oriented research (Behring Reference Behring1893b, 307–309). Nevertheless, Behring tried to defuse the theoretical dispute with Metschnikoff by viewing themselves as engaged in the marshal competition necessary to ensure scientific progress: He proposed that they see each other as two different (but opposed) parties, locked in a concerted struggle against diphtheria.Footnote ⁴⁰ Subsequent letters continued this discussion of the differences between their concepts of immunity, but also contained proposals for a kind of “ceasefire.” In one letter, Behring wrote: “Esteemed colleague, if for the present we two, as representatives of two contrary standpoints, still seem to work against each other, if we actually still wage a war, then I nevertheless wish that you, like me, never lose sight of the possibility of establishing common ground for a peace agreement” (Behring to Metschnikoff, 29 March 1894, in AIP, Mtc 2, Doc. 27902). Regardless of their theoretical differences, Behring was not too orgulous to ask for practical advice, when in the spring of 1894 he had run into difficulties in maintaining the high immunizing power of the sera produced in partnership with Farbwerke Hoechst (Throm Reference Throm1995, 52). Faced with this problem, he even asked Metschnikoff if he could get information from Roux about how to produce highly potent toxin in order to force the immunization process forward. And Roux obliged, describing how to prepare a very powerful toxin within a few weeks by constantly aerating the diphtheria cultures.Footnote ⁴¹

Thus, we can see that the scientific interest of Roux, Metschnikoff, and Behring in each other's work and their mutual adoption of experimental practices were not inhibited by their theoretical or conceptual differences. In later years, Behring even publicly acknowledged the validity of phagocytosis, although he did point out that it was not as vitally important as the toxin-antitoxin reaction as a mechanism in immunity (Behring Reference Behring and Eulenburg1896). Over the years, the relationship between the protagonists actually became amicable and Behring named both Roux (in 1897) and Metschnikoff (in 1906) as godfathers of two of his children (Zeiss and Bieling Reference Zeiss and Bieling1941, 218). In the next section I will show how distinctions between the concepts of immunity had practical consequences for the methods of evaluating the efficacy of the diphtheria serum.

Serum Evaluation

On several different occasions in the beginning of his research on diphtheria serum, Behring had insisted that the antitoxin theory was mainly of “heuristic value” for him, both in extracting the therapeutic serum and in measuring its efficacy (Behring Reference Behring1892, 2).Footnote ⁴² The antitoxin hypothesis was thus also associated with a rational method for evaluating the efficacy of the serum. Behring carefully elaborated his evaluation techniques parallel to the diphtheria serum research, as can easily be surmised from his publications: In 1891, he adopted one of Ehrlich's methods (Behring Reference Behring1891, 1395), which he completely changed in 1892 in collaboration with Wernicke (Behring and Wernicke Reference Behring and Wernicke1892) and again in 1893, this time working together with Oscar Boer (Behring, Boer, and Kossel Reference Behring, Boer and Kossel1893), before a partnership with Ehrlich gave rise to a refined and standardized method at the end of 1893 (Behring Reference Behring1893c, 281–282; Behring and Boer Reference Behring and Boer1894). The method published by Behring and Wernicke in 1892 measured the serum in proportion to the weight of guinea pigs that remained free of symptoms. The serum was injected 24 hours before the guinea pigs were infected with a dose of diphtheria culture strong enough to kill an untreated animal (Behring and Wernicke Reference Behring and Wernicke1892, 16–18). A revision of the evaluation method in the spring of 1893 by Behring and Boer introduced several changes: Instead of diphtheria culture, a pure toxin was injected a mere fifteen minutes after the serum (at different points of the body). The quality of the serum was now no longer measured by its capacity to prevent symptoms, but by its capacity to prevent the test animal's death after it had been given a lethal dose. Furthermore, the strength of a serum being tested now had to be specified in relation to a so called “normal serum” defined by Behring as a therapeutically effective dose for humans. Its actual strength was assessed against a toxin of confirmed high potency that Behring had extracted from two-year-old diphtheria cultures. This poison was later called the “normal toxin” (Behring and Boer Reference Behring and Boer1894, 455). According to Ehrlich's and Behring's new technique, the test-serum was no longer injected in advance, but was mixed with the toxin in vitro before injection into the animal. The standard toxin was now made and defined by Behring and Ehrlich, and they asserted that no one had the right to reference their “titer” without express permission (Behring and Ehrlich Reference Behring and Ehrlich1894).

This early determination of effective methods of evaluation, as well as the subsequent revision and standardization applied to the techniques, helped to stabilize Behring's experimental system: First, an effective evaluation method enabled experimental results to be compared over a longer time period so that the effects of variations in experimental conditions could be assessed. Secondly, a network of strategic cooperation between bacteriological research, clinicians, pharmaceutical enterprises, and state authorities developed around the serum: the concept of the serum's “value” bridged the differences between these diverse interest groups because it was broad and flexible enough to be translated from a laboratory to a therapeutic measure and because it assumed an economic and a public health value.Footnote ⁴³ Defining a “normal serum” based on this “value” provided a metric to easily calculate the therapeutic value of sera and conduct cost-benefit analyses of large-scale production processes. Indeed, Behring was enough of a businessman to recognize the advantages of having a metric for the “value” of the serum and of course the Farbwerke Hoechst were eager to have such a useful accounting tool (Behring Reference Behring1893c, 287).Footnote ⁴⁴ Having standard units of “value” also demonstrated the public health effects of serotherapy to state officials (Hüntelmann Reference Hüntelmann2006, 279–281). Third, the revised methods of 1893 set a standard that made it very difficult for competitors in the serum (research) market to demonstrate the quality of their serum to the scientific community. Thus, it was probably no accident that Behring redefined the evaluation method at the same time that Wernicke and Aronson were presenting their dog-sera. Aronson in particular threatened Behring's ambitious plans because he too was aiming to enter the private sector. Considering Wernicke's publication on dog-sera, it is noteworthy that he felt obliged to reassess the value of his sera based on the newly developed method, because it was viewed as being more rational and exact. This meant that he had to measure using Behring's “normal toxin” (Wernicke Reference Wernicke1893a, 232–234). Wernicke carried out this re-evaluation during the spring of 1893 and thus couldn't submit his findings to the Archiv für Hygiene until May of 1893, two months after Behring had published a report of his new and enhanced sera (Behring, Boer, and Kossel Reference Behring, Boer and Kossel1893, 391). In 1894, Behring used articles in the scientific press to attack the therapeutic serum that was now produced by Schering and Aronson. On the basis of his evaluation method, Behring tried to denigrate the serum and to call Aronson's scientific credentials into question, alleging that he had provided unreliable specifications for the “therapeutic value” of Schering's serum and was motivated only by its potential “monetary value” (Behring Reference Behring1894a; Behring Reference Behring1894b). Because of the technical problems involved in obtaining a poison of uniform toxicity, other serum researchers depended on the “normal toxin” if they wanted to match the widely recognized evaluation standards set up by Behring and Ehrlich. In addition, their method became institutionalized once a central state institute was established in the German Reich in 1895 (Hüntelmann Reference Hüntelmann2006). The institute was responsible for inspecting the quality of diphtheria sera and turned the evaluation technique into an obligatory point of passage (Callon Reference Callon and Law1986) for serum producers.

Unlike his German colleagues, Roux was under no pressure to maneuver through such a point of passage, primarily because in France value comparison was of less eminent importance: there it did not serve to control competing providers because the Pasteur Institute in Paris held a monopoly over serum-production.Footnote ⁴⁵ However, for the sake of comparison with the German sera, Roux made the effort to measure the serum using one of the accepted evaluation methods. He decided on the method published by Behring and Wernicke in 1892 (Roux and Martin Reference Roux and Martin1894, 620), which he and Vaillard had already used in a modified form for measuring tetanus serum (Roux and Vaillard Reference Roux and Vaillard1893, 74).Footnote ⁴⁶ Nevertheless, in 1894 Roux launched a fundamental critique of Behring's methods of quantitative evaluation, arguing that “the living animal itself was a measuring instrument that differed from trial to trial and that was equipped with its individual sensibility” (Roux and Martin Reference Roux and Martin1894, 620). Above all, he was not convinced of the appropriateness of the in vitro mixing method employed by Behring and Ehrlich, because he considered even serum-induced immunity to involve the active acquisition of immunity through phagocytosis. This meant that for Roux and his collaborators the living body and its ability to respond was an obligatory point of passage between the injection of the serum and the infection. On the other side of the Franco-Prussian border, Behring and his antitoxin hypothesis could basically rationalize away the vital variability of the guinea pigs. His ideal test animal was not regarded as an organism which reacted individually, but merely as a standardized indexing device.Footnote ⁴⁷