Fig. 40. Fig. 41. Fig. 42.
Fig. 40.—Phagocyte of a guinea-pig in the course of engulphing a very mobile undulating spirillum. Fig. 41.—The same, forty minutes later. Fig. 42.—The same taken half an hour after Fig. 41. (From Metschnikoff’s “Immunity.”)
Fig. 43.
A large kind of phagocyte of the guinea-pig, killed and stained for microscopic examination. It shows the large spherical nucleus and three specimens of the spirillum of relapsing-fever which have been engulphed, and are lying within its protoplasm. They would have been slowly digested—that is to say, dissolved by the digestive juices within the phagocyte. (From Metschnikoff’s “Immunity.”)
Disease-germs, bacteria, or protozoa produce poisons which sometimes are too much for the phagocytes, poisoning them and so getting the upper hand. But, as Metschnikoff showed, the training of the phagocytes by weak doses of the poison of the disease-germ, or by weakened cultures of the disease-germ itself, brings about a power of resistance in the phagocytes to the germ’s poison, and thus makes them capable of attacking the germs and keeping them at bay. Hence the value of inoculations.
The discussion and experiments arising from Metschnikoff’s demonstrations have led to the discovery of the production by the phagocytes of certain exudations from their substance which have a most important effect in weakening the resistance of the intrusive bacteria and rendering them easy prey for the phagocyte. These are called ‘sensitisers,’ and have been largely studied. They may be introduced artificially into the blood and tissues so as to facilitate the work of the phagocytes, and no doubt it is a valuable remedial measure to make use of such sensitisers as a treatment. Dr. Wright considers that such sensitisers are formed in the blood and tissues independently of the phagocytes, and has called them ‘opsonins,’ under which name he has made most valuable application of the method of injecting them into the body so as to facilitate the work of the phagocytes in devouring the hostile bacteria of various diseases. Each kind of disease-producing microbe has its own sensitiser or opsonin; hence there has been much careful research and experiment required in order to bring the discovery into practical use. Metschnikoff himself holds and quotes experiments to show that the ‘opsonins’ are actually produced by the phagocytes themselves. That this should be so is in accordance with some striking zoological facts, as I pointed out nearly twenty years ago.[20] For the lowest multicellular animals provided with a digestive sac or gut, such as the polyps, have that sac lined by digestive cells which have the same amœboid character as ‘phagocytes,’ and actually digest to a large extent by swallowing or taking into their individual protoplasm raw particles of food. Such particles are enclosed in a temporary cavity, or vacuole, into which the cell-protoplasm secretes digestive ferment and other chemical agents. Now there is no doubt that such digestive vacuoles may burst and so pour out into the polyp’s stomach a digestive juice which will act on food particles outside the substance of the cells, and thus by the substitution of this process of outpouring of the secretion for that of ingestion of food particles into the cells we get the usual form of digestion by juices secreted into a digestive cavity. Now this being certainly the case in regard to the history of the original phagocytes lining the polyp’s gut, it does not seem at all unlikely, but on the contrary in a high degree probable, that the phagocytes of the blood and tissues should behave in the same way and pour out sensitisers and opsonins to paralyse, and prepare their bacterial food. And the experiments of Metschnikoff’s pupils and followers show that this is undoubtedly the case. Whether there is any great variety of and difference between ‘sensitisers’ and ‘opsonins’ is a matter which is still the subject of active experiment. Metschnikoff’s conclusion, as recently stated in regard to the whole progress of this subject, is that the phagocytes in our bodies should be stimulated in their activity in order successfully to fight the germs of infection. Alcohol, opium, and even quinine, hinder the phagocytic action; they should therefore be entirely eschewed or used only with great caution where their other and valuable properties are urgently needed. It appears that the injection of blood-serum into the tissues of animals causes an increase in the number and activity of the phagocytes, and thus an increase in their resistance towards pathogenic germs. Thus Durham (who was a pioneer in his observations on the curious phenomena of the ‘agglutination’ of blood corpuscles in relation to disease) was led to suggest the injection of sera during surgical operations, and experiments recently quoted by Metschnikoff seem to show that the suggestion was well founded. Both German and French surgeons have employed the method with successful results, and the demonstration that an immense number of microbes are thus taken up and destroyed by the multiplication (due to their regular increase by cell-division) of the phagocytes of the injected patient. After years of opposition bravely met in the pure scientific spirit of renewed experiment and demonstration, Metschnikoff is at last able to say that the foundation-stone of the hygiene of the tissues—the thesis that our phagocytes are our arms of defence against infective germs—has been generally accepted.