The faculty possessed by the leucocytes of seizing solid corpuscles coming in contact with them, inglobing them, and absorbing them, or, as M. Metchnikoff calls it, intracellular digestion or phagocytosis, is easily observed. If we mix fine granulations of carmine or cinnabar, mingled with slightly salted water, with a drop of lymph, we can see the coloring matter penetrating the leucocytary protoplasmic mass, which is soon stuffed with it. The anatomo-pathologists had already observed, in tattooed subjects, white globules charged with grains of charcoal or vermilion. It is legitimate to conclude that some parts of the coloring matter that had been introduced under the epidermis had been taken up by the white globules. This proceeding has been observed in the very act by M. Metchnikoff.
A classic experiment illustrating this operation is now common in our laboratories, and the fact of phagocytosis has come to be regarded as incontestable.
The generality of the phenomenon results from the leucocyte preserving its phagocytic faculty in all its peregrinations, and these peregrinations are unlimited. The tendency of the nomadic elements to push on and insinuate themselves into the finest interstices and the narrowest passages is a rudiment of a tactile sense, to this extent simply a physical phenomenon, which MM. Mascart and Bordet have clearly distinguished. As soon as a leucocyte touches a resisting body it reacts to the contact by applying the largest possible surface to it. It spreads out, becomes thin, stretches itself along, and ceases deforming itself only after it has obtained the maximum of contact. By such mechanism it penetrates objects that offer it any breach and overcomes them. When the foreign body has been disaggregated into fragments, into small enough grains, phagocytosis intervenes and disposes of the remains. In this way the organism sometimes rids itself of splinters of bone that remain in the tissues after a fracture. So, too, the leucocytes, when occasion arises, repair the blunders of surgeons by extracting and absorbing forgotten objects left in wounds, while at other times they act as auxiliaries by destroying things that have been voluntarily abandoned in them, like threads of catgut in buried sutures and drains of decalcified bone.
There are two conditions, under normal circumstances, in which phagocytosis plays a marked part. The first is the case where vital action brings on the destruction of the organs or the tissues, or, to use exact language, their disintegration in a solid form. The wastes of organic activity are usually in liquid form, and, turned into the blood, they are eliminated in that state through the natural emunctories. Sometimes, however, disintegration results in solid wastes, and the phagocytes do the work of carrying them away. This is the case with the red globules of the blood, which, after a longer or shorter career, are deposited in the spleen and break up into débris, some of the parts of which are insoluble in the interstitial liquids. The leucocytes collect around these residues so thickly as sometimes to fuse themselves into a solid mass, a sort of plasmodium or giant cell which digests the débris. At other times, and more rarely the isolated leucocytes are not able to absorb the incorporated matters. They then conduct them to the surface of the intestine and discharge them there. A like phenomenon occurs in the liver. The coloring matter of the blood frequently gives rise to insoluble ferruginous deposits which the leucocytes have to convey to the digestive tube. This occurs when a wound provokes an effusion of blood and a mortification of the red globules or of the neighboring anatomical elements. All of the waste that can not take the liquid form and pass in that condition into the circulatory passages is incorporated within the phagocytes. The mechanism of resorption of bone does not seem different.
The phagocytes perform a similar function in another process which very frequently takes place in various animals that pass through metamorphoses, as in insects whose organs are transformed in changing from one stage of their existence to another, and in tadpoles which lose their tails in becoming frogs; the old parts that disappear are devoured by phagocytes.
Especially in the case of infectious diseases has the protective part performed by the leucocytary phagocytes been brought into full view by M. Metchnikoff. He has shown that the white globules rush to meet the bacterides of inflammation that are introduced through any wound, absorb them, and render them powerless to do harm. In the lymphatic organs—the spleen, the lymphatic ganglions, and the marrow—the white globules are normally accumulated, and there is where the struggle is most active between the bacterides of inflammation which are swarming in the blood and the defensive agents of the organism. The same takes place with the spirilla of recurrent typhus and the microbe of erysipelas.
The leucocytes are capable of adapting themselves to conditions different from those in which they usually live, provided the change is not too abrupt. It may sometimes occur that the poison secreted by a microbe will paralyze and kill the leucocyte, unless care has been taken, by inoculations of virus, at first attenuated and afterward gradually increasing in virulence, to create an immunity in the phagocyte, to make it refractory to the poison and capable of swallowing the toxic bacterium without suffering from it. Explanations have been sought in this property for the virtue of vaccination and the immunity that results from it, but they are evidently only fragmentary, and there are other theories of immunity.
The leucocytes are not always victorious over the microbes, and when these excel in numbers or force it sometimes comes to pass that they are overcome and succumb. Poisoned by the substance they have incorporated, they undergo a fatty degeneration and become globules of pus. Pus is therefore formed of the cadavers of conquered leucocytes. Although that humor ought, for the good of the system, to be rejected, like every other mortified part, it is nevertheless true that the production of it is a beneficent effort, and a salutary reaction of Nature against the morbid agent.
It will be an enduring honor to the name of M. Metchnikoff that he has revealed the importance of the function of phagocytes, and has enriched science with a large number of new truths. A part of this honor will be reflected upon the Pasteur Institute, which has welcomed the eminent biologist for many years, and has intrusted the direction of one of its services to him. The learned Russian, in creating the study of phagocytism, with its causes, mechanism, and consequences, has opened a very extensive field of research to which we have given only a distant and cursory glance.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.