CHAPTER XXII
IMMUNITY AND CURATIVE INOCULATIONS
During the last twenty years the whole attitude of the study and investigation of disease-causing microbes has advanced from the preliminary step of merely identifying certain microbes as the causes of certain diseases to a further step, viz. that of attempting to defend the animal and the human body against their attacks in the manner already so finely started by Pasteur. For many years disease after disease was examined and found to be caused by special bacteria or other microbes. Even non-infectious diseases or diseases only communicable under very special conditions were found to be due to microbes, so that it is probable that all disease that is not due to congenital malformation or to mechanical injury, or to poison fabricated in the weapons of larger animals and plants, or by man himself, is due to microbes. "Life," says Lord Justice Moulton, "is one ceaseless war against these enemies, and the periods of our too-transient successes are known as health." One of the last diseases traced to microbes is that sad condition known as "infantile paralysis," by which so many of the brightest and best members of the community have been crippled, from childhood onwards, through life.
Of late we have been making rapid strides in arriving at a knowledge as to how Nature herself protects higher creatures from the excesses and exuberance of destructive microbes, and we are now able to see that it is in adopting her methods that our best hope of increasing that protection lies. Nature is satisfied if the efficacy of her defence is sufficient to save enough individuals to carry on the race. Man desires in the case of his own fellows to out-do Nature and to save all.
A century and a half ago, before the true character of infective disease was understood, it was observed that an individual who was attacked by the smallpox and recovered became incapable of receiving the infection again. He was "protected" or "immune." The practice of "inoculation" was introduced from the East by Lady Montague. The infectious matter was introduced from a smallpox patient into the person to be protected by rubbing it into a scarified part of the skin. A much less severe attack of smallpox was thus produced than that which usually followed the natural infection, which (though we do not know precisely its mode of entrance) is more widely spread through the blood. At the same time the condition of "immunity" after the attack was brought about with equal efficacy. When Jenner introduced inoculation with "cowpox" for the purpose of establishing "immunity" in the vaccinated person, inoculation with smallpox itself was a very usual practice. It was open to the objection that sometimes an unexpectedly violent attack of the disease was produced, resulting in death, and that the active infection was kept alive and ever present in the community. The notion with regard to the mode in which "immunity" was produced by either the Montacutian or Jennerian inoculation was, even after the general knowledge of microbes as the living contagion of disease had been arrived at, that the mild attack due to inoculation "used up" something in the blood—in fact, exhausted the soil, so that the infective matter or microbe could no longer flourish in the blood. And this view was accepted as the explanation of the "immunity" to the anthrax disease conferred on cattle and sheep by Pasteur's inoculations of weakened, but still actively growing, cultures of the anthrax bacillus. Another theory was that they produced something in the blood by their own life-processes which checked their further growth, just as yeast will not grow in wort in which it has produced 8 per cent. of alcohol, and as a fire may be choked by its own smoke or ashes.
We now know that both these explanations of "immunity" are incorrect. Nature provides at least three varieties of defence within the blood of higher animals against disease-producing microbes which have broken through the outer line of fortification, the skin. These three methods are effective in different cases (one in this disease, the other in that), and, on the whole, are sufficient to preserve the races of animals (including man) from complete destruction. These are (1) the production in the blood of an antidote to the toxin or poison elaborated by the invading microbe—an antitoxin, which chemically neutralises the toxin; (2) the production in the blood of the attacked animal of a "germicidal" poison which repels and kills the attacking microbes themselves (not merely neutralising their poisonous products); (3) the extermination of the intrusive, disease-producing microbes by a kind of police, which scour the blood channels and tissues and "eat up"—actually engulf and digest—the hostile intruders. These latter agents, actual particles of the living animal in which they exist, are the "eater-cells," or "phagocytes"—minute, viscid, actively moving cells, resembling the animalcules called "amœba." They are only the one two-thousandth of an inch in diameter, and are known as the white or colourless corpuscles of the blood. They are far less numerous than the red blood-corpuscles, which are the agents for carrying oxygen, but there are eight thousand million of them in a large spoonful of blood. They are the really important agents in protecting us from microbes, since they not only engulf and digest and so destroy those intruders, but it is probable (not certain) that they also are the manufacturers of the antitoxins and of the germicidal poisons.
If these three defensive processes given us by Nature are in working order, that is to say, if we are "healthy," they should secure to us a sufficient "immunity"—at at any rate, "recovery"—from any attack of disease-producing microbes. But they are not in "unselected," widely ranging mankind always equal (in their unaided natural state) to their task.
The attempts to produce immunity by vaccination with weakened or localised disease germs is really an attempt to train and develop to a high point the activities of the phagocytes or eater-cells of the blood.
The introduction of antitoxins by injection of them into the blood (as in the treatment of diphtheria, lock-jaw, and snake-bite) is an attempt to bring to the rescue of a patient who would sooner or later produce his own antitoxins (but perhaps too late or in insufficient quantity) the similar antitoxin obtained from the blood of another animal which has been artificially made to produce in its blood an excessive quantity of that substance.