accomplishment are detailed in “the big red book,” Smallpox and Its Eradication (13).
Figure 3 Triomphe de la Petite Vérole (Triumph of Smallpox). Vaccination was feared on the European continent as well as in England. This French caricature satirized that fear. It shows a woman with smallpox turning into a mermaid, a physician riding a cow, and an apothecary with a giant syringe pursuing frightened children. (Courtesy of the Wellcome Library, London, United Kingdom.)
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Start of the Science of Immunology: Phagocytosis and Humoral Immunity
Theories of humoral immunity, the basis of serological testing, were forged against Elie Metchnikoff’s (Fig. 4) theory of phagocytosis in host defense. Metchnikoff’s studies of phagocytosis evolved from his interest in digestion by invertebrates in the latter 1870s (40). In her biography of Metchnikoff, Olga, his second wife, gives a charming quote describing the inception of her husband’s phagocyte theory: “One day when the whole family had gone to a circus to see some extraordinary performing apes, I remained alone with my microscope, observing the life in mobile cells of a transparent star-fish larva, when a new thought suddenly flashed across my brain. It struck me that similar cells might serve in the defense of the organism against intruders” (27). Metchnikoff devised a simple experiment in which he “introduced them (rose thorns) at once under the skin of some beautiful star-fish larvae as transparent as water.” The following morning he confirmed that the thorns were surrounded by mobile cells. “That experiment formed the basis of the phagocyte theory, to the development of which I devoted the next twenty-five years of my life.”
Figure 4 Elie Metchnikoff. Metchnikoff’s studies of phagocytosis initiated the science of immunology, specifically cellular immunity. With Paul Ehrlich, who developed the theoretical basis for the action of antibodies or humoral immunity, Metchnikoff received the Nobel Prize in 1908. (Courtesy of Wikimedia Commons.)
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On showing his experiments to Rudolf Virchow, the father of modern pathology, Metchnikoff was advised to proceed with caution. As opposed to Metchnikoff’s view of inflammation as “a curative reaction,” contemporary medicine viewed leukocytes as supporting the growth of microbes (27). Metchnikoff left Messina and on subsequent travels through Vienna, the word “phagocytes” was suggested by zoologists as a Greek translation of “devouring cells” (27).
In 1884, Metchnikoff published a seminal work on phagocytosis in Virchow’s Archive (26). Regretfully, however, as his wife recorded, “. . . the memoir passed unnoticed; the full significance of it had not been grasped” (27). Over a century later, A. M. Silverstein, in his history of immunology, amplified that observation: “One may conclude that the cellular theory of immunity advanced by Elie Metchnikoff in 1884 did not constitute just one further acceptable step in a well-established tradition; rather it represented a significant component of a conceptual revolution with which contemporary science had not yet fully learned to cope” (40).
As described in the 1884 work, Metchnikoff investigated a fungal disease of Daphnia, a transparent water flea (22). Daphnia ingested asci, sac-like structures in which spores are formed. Upon release into the digestive tract, spores traversed the intestinal wall to the body cavity, where they were attacked by blood cells, ultimately disintegrating into granules. Host giant cells were seen to have formed from the fusion of ameboid cells; however, the host did not always win out.
Using Metchnikoff’s other studies of anthrax bacilli inoculated subcutaneously into frogs and rabbits as a take-off point, George Nuttall confirmed that phagocytosis was observed but also noted extracellular destruction of anthrax (reference 30, translated in reference 22). With blood and other body fluids in vitro from several different species of animals, either immunized or not, Nuttall demonstrated degeneration of free anthrax bacilli and other bacteria. Hence, phagocytosis was not the entire explanation for destruction of bacteria. “These investigations have shown that independently of leucocytes, blood and other tissue fluids may produce morphological degeneration of bacilli” (22). The studies of Metchnikoff on phagocytosis and Nuttall on humoral mechanisms demonstrated the two arms of host defense. However, they did not demonstrate immunization, specific host responses after particular challenges. That was most clearly achieved in studies of immunity to tetanus and diphtheria toxins by Emil Behring and Shibasaburo Kitasato.
In his masterful The History of Bacteriology, published in 1938, William Bulloch succinctly described the stepwise series of discoveries leading up to the landmark work of Behring and Kitasato (8): “F. Loeffler (1884) discovered the diphtheria bacillus and Kitasato (1889) proved that the tetanus bacillus is the cause of lockjaw. Roux and Yersin (1889) proved that the diphtheria bacillus operates by virtue of a poison (toxin) which it elaborates, and K. Faber (1889) showed that the tetanus bacillus acts in a similar manner.” The stage was set for Behring and Kitasato to describe the mechanism of immunity to diphtheria and tetanus (4). Observing that blood from immune animals neutralized diphtheria toxin, they used this concept to devise experiments to demonstrate that treated animals would be insensitive to tetanus. These experiments are described in Behring and Kitasato’s paper translated in Brock’s collection of landmark papers in microbiology (7). They present experiments in support of four conclusions: (i) blood of a rabbit immune to tetanus neutralizes or destroys tetanus toxin, (ii) that property is found in cell-free serum, (iii) the property is stable in other animals when used for therapy, and (iv) the property does not occur in animals not immune to tetanus. The importance of the paper can be found in Brock’s comment, “The science of serology can be said to have begun with this paper.” Behring followed this paper with a similar report focusing on diphtheria (3). He won the Nobel Prize in 1901.
While Silverstein characterized the 1890 Behring and Kitasato paper as “[t]he most telling blow to the cellular theory of immunity . . .” (40), the interplay of phagocytic and humoral mechanisms was studied by A. E. Wright and S. R. Douglas. They presented experiments allowing them to state, “We have here conclusive proof that the body fluids modify the bacteria in a way which renders them ready prey to the phagocytes . . . We may speak of this as an opsonic effect.” They termed the elements in the blood exerting this effect as “opsonins” (
