Immunity present in an animal and not due to human interference is to be regarded as natural immunity, while if brought about by man’s effort it is considered artificial. Those cases of natural immunity mentioned above which are common to divisions, classes, orders, families, species or races of organisms and to those few individuals where no special cause is discoverable, must be regarded as instances of true inheritance through the germ cell as other characteristics are. All other kinds of immunity are acquired. Occasionally young are born with every evidence that they have had a disease in utero and are thereafter as immune as though the attack had occurred after birth (“small-pox babies,” “hog-cholera pigs”). Experiment has shown that immune substances may pass from the blood of the mother to the fetus in utero and the young be immune for a time after birth (tetanus). This is of no practical value as yet. It is a familiar fact that with most infectious diseases recovery from one attack confers a more or less lasting immunity, though there are marked exceptions.
Active Immunity.—By active immunity is meant that which is due to the actual introduction of the organism, or in some cases of its products. The term active is used because the body cells of the animal immunized perform the real work of bringing about the immunity as will be discussed later. In passive immunity the blood serum of an actively immunized animal is introduced into a second animal, which thereupon becomes immune, though its cells are not concerned in the process. The animal is passive, just as a test-tube, in which a reaction takes place, plays no other part than that of a passive container for the reagents.
In active immunity the organism may be introduced in what is to be considered a natural manner, as when an animal becomes infected, has a disease, without human interference. Or the organism may be purposely introduced to bring about the immunity. For certain purposes the introduction of the products of the organism (toxins) is used to bring about active immunity (preparation of diphtheria and tetanus antitoxin from the horse). The method of producing active immunity by the artificial introduction of the organism is called vaccination, and a vaccine must therefore contain the organism. Vaccines for bacterial diseases are frequently called bacterins. The use of the blood serum of an immunized animal to confer passive immunity on a second animal is properly called serum therapy, and the serum so used is spoken of as an antiserum, though the latter word is also used to denote any serum containing any kind of an antibody ([Chapters XXVII–XXXI]). In a few instances both the organism and an antiserum are used to cause both active and passive immunity (serum-simultaneous method in immunizing against hog cholera).
In producing active immunity the organism may be introduced (a) alive and virulent, but in very small doses, or in combination with an immune serum, as just mentioned for hog cholera. The introduction of the live virulent organism alone is done only experimentally as yet, as it is obviously too dangerous to do in practice, except under the strictest control (introduction of a single tubercle bacillus, followed by gradually increasing numbers—Barber and Webb). More commonly the organisms are introduced (b) alive but with their virulence reduced (“attenuated”) in one of several ways: (1) By passing the organism through another animal as is the case with smallpox vaccine derived from a calf or heifer. This method was first introduced by Jenner in 1795 and was the first practical means of preventing disease by vaccination. This word was used because material was derived from a cow—Latin vacca. (2) By drying the organism, as is done in the preparation of the vaccine for the Pasteur treatment of rabies, where the spinal cords of rabbits are dried for varying lengths of time—one to four days, Russian method, one to three days, German method, longer in this country. (It is probable that the passage of the “fixed virus” through the rabbit is as important in this procedure as the drying, since it is doubtful if the “fixed virus” is pathogenic for man.) It would be more correct to speak of this as a preventive vaccination against rabies, since the latter is one of the few diseases which is not amenable to treatment. The patient always dies if the disease develops. (3) The organism may be attenuated by growing at a temperature above the normal. This is the method used in preparing anthrax vaccine as done by Pasteur originally. (4) Instead of growing at a higher temperature the culture may be heated in such a way that it is not killed but merely weakened. Black-leg vaccines are made by this method. (5) Chemicals are sometimes added to attenuate
the organisms, as was formerly done in the preparation of black-leg vaccine by Kruse’s method in Germany. The use of toxin-antitoxin mixtures in immunizing against diphtheria and in the preparation of diphtheria antitoxin from horses is an application of the same principle, though here it is the product of the organism and not the organism whose action is weakened. (6) Within the past few years the workers in the Pasteur Institute in Paris have been experimenting with vaccines prepared by treating living virulent bacteria with antisera (“sensitizing them”) so that they are no longer capable of causing the disease when introduced, but do cause the production of an active immunity. The method has been used with typhoid fever bacilli in man and seems to be successful. It remains to be tried out further before its worth is demonstrated (the procedure is more complicated and the chance for infection apparently much greater than by the use of killed cultures). The term sero-bacterins is used by manufacturers in this country to designate such bacterial vaccines. (7) Growing on artificial culture media reduces the virulence of most organisms after a longer or shorter time. This method has been tried with many organisms in the laboratory, but is not now used in practice. The difficulties are that the attenuation is very uncertain and that the organisms tend to regain their virulence when introduced into the body.
In producing active immunity against many bacterial diseases the organisms are introduced (c) dead. They are killed by heat or by chemicals, or by using both methods ([Chapter XXX]).
When the products of an organism are introduced the resulting immunity is against the products only and not against the organism. If the organism itself is introduced there results an immunity against it and in some cases also against the products, though the latter does not necessarily follow. Hence the immunity may be antibacterial or antitoxic or both.
Investigation as to the causes of immunity and the various methods by which it is produced has not resulted in the discovery of specific methods of treatment for as many diseases as was hoped for at one time. Just at present progress in serum therapy appears to be at a standstill, though vaccines are giving good results in many instances not believed possible a few years ago. As a consequence workers in all parts of the world are giving more and more attention to the search for specific chemical substances, which will destroy invading parasites and not injure the host (chemotherapy). Nevertheless, in the study of immunity very much of value in the treatment and prevention of disease has been learned. Also much knowledge which is of the greatest use in other lines has been accumulated. Methods of diagnosis of great exactness have resulted, applicable in numerous diseases. Ways of detecting adulteration in foods, particularly foods from animal sources, and of differentiating proteins of varied origin, as well as means of establishing biological relationships and differences among groups of animals through “immunity reactions” of blood serums have followed from knowledge gained by application of the facts or the methods of immunity research. Hence the study of “immunity problems” has come to include much more than merely the study of those factors which prevent the development of disease in an animal or result in its spontaneous recovery. A proper understanding of the principles of immunity necessitates a study of these various features and they will be considered in the discussion to follow.