As it was important to determine whether the human subject was capable of acquiring a certain immunity by absorbing antibodies contained in the milk, the study of this question was taken up, especially from the point of view of agglutinative power. Although the relations of this agglutinative power with immunity are very problematical it would be interesting, bearing in mind the analogy between the agglutinative, antitoxic, and protective properties, to ascertain whether the ingestion of agglutinative milk can confer any agglutinative property on the blood serum. Numerous researches in this direction were carried out in connection with typhoid fever. Widal and Sicard (l.c.) caused a person to drink daily (for a period of three weeks) half a litre of milk coming from an immunised goat, a milk which powerfully agglutinated the typhoid coccobacillus. The blood, examined on several occasions, never showed the slightest agglutinative power. This experiment goes to prove that, in the adult human subject, the agglutinin does not pass from the alimentary canal into the circulation. May it not perhaps be otherwise in infants which are fed on milk only? An observation by Landouzy and Griffon[761] seemed to confirm this supposition. They first demonstrated the agglutinative power of the blood serum in a woman who had contracted typhoid fever three months after her lying-in. Being a mild attack the woman continued to suckle her child during the whole course of the fever. On examination of the blood of the infant it was found that the serum agglutinated the micro-organism of typhoid fever. These observers did not measure the agglutinative power of the blood, either in the infant or in the mother. This omission deprives their observation of value since it is now recognised that normal human blood fairly frequently exhibits some power of agglutinating the typhoid coccobacillus. For diagnostic purposes it is necessary, therefore, always to measure this power in order to be sure that it is higher than that of the normal blood.
It is all the more difficult to draw any positive conclusion from the observations of Landouzy and Griffon because in several similar cases the result has been entirely different. Thus Achard and Bensaude[762] have shown that the blood of an infant, suckled by a nurse attacked by typhoid fever and whose serum became distinctly agglutinative, was incapable of bringing about clumping of the typhoid coccobacilli. Schumacher[763], working in Fraenkel’s laboratory in Halle, studied a case with very great care. A woman gave birth at full term to an infant whose blood serum exhibited a certain agglutinative power. The mother suckled the infant from its birth. Although her milk manifested a very considerable agglutinative property, the blood of the child exhibited not only no increase in agglutinative power but a marked diminution. The agglutinin of the maternal blood had not passed into the fluids of the child.
From the point of view of the impossibility of acquiring immunity by suckling, therefore, the human subject may be grouped with the guinea-pig, rabbit and cat. Up to the present the mouse is the only exception. It would be very important, with the object of finding a means of communicating immunity by way of the intestine, to study the precise conditions which govern this phenomenon. In hereditary immunity, or rather in what appears to be such, those cases where the new-born animal exhibits a resisting power induced by the vaccination to which it has been subjected in the womb of the mother must be borne in mind. We have already cited the example given by Remlinger of rabbits and guinea-pigs born refractory against the typhoid coccobacillus, which had been injected into the mother animals. In those cases where the vaccination of the mothers has been carried out during the period of gestation the immunity of the young ones is more permanent than when it was completed before that period. Into this same group come those cases where women, vaccinated during the course of pregnancy, give birth to infants refractory to vaccine. Similar facts have been reported by veterinary surgeons with regard to sheep-pox; Arloing, Cornevin, and Thomas[764] have offered similar demonstrations with regard to symptomatic anthrax.
These results may be more or less closely associated with those where the child attacked by an infective disease immunises the mother. Such facts are rare. We know that a healthy mother may give birth to a syphilitic child; the affected father introducing the virus with the sperm, the contaminated foetus contracts the disease and the new-born infant is syphilitic. According to Ehrlich and Hubener (l.c. p. 54), the foetus instead of infecting the mother sets up in her a refractory condition. It must be confessed that as yet we do not understand the mechanism of this immunity; but in any case we have here to do with an example of immunity naturally acquired under very special conditions.
Here again must be recognised another form of immunisation:—where the child born of a syphilitic mother remains healthy and contracts syphilis neither by the breast nor through the kisses of the mother. Here, undoubtedly, we have an immunity against syphilis acquired in the womb of the mother, who may, however, readily communicate her disease to other persons by means which are without effect on her own infant. This example comes under the law of Profetta. Here again the mechanism of the acquired immunity is absolutely unknown.
It must be admitted that, generally, we are still very imperfectly informed concerning immunity as acquired by natural paths. In cases where this immunity is developed as the result of an attack of an infective disease the phenomena observed closely resemble those that have been observed after vaccination by living, fully active, or attenuated viruses, by micro-organisms which have been killed, or by the products of these micro-organisms. These vaccinations which bring about isopathic (von Behring) or active (Ehrlich) immunity give rise to transient and mild diseases and are confined almost completely to the diseases contracted by natural means which terminate in recovery and give rise to a refractory condition. The immunisation of the foetus comes into the same series.
On the other hand, the immunity which was believed to be hereditary and which results merely from the direct passage of the antibodies of the blood or of the milk of the mother to the foetus and to the child come into a group of cases characterised by what Ehrlich has termed a condition of passive immunity. We have already discussed (Chapter X) the thesis that this term “passive” is applicable only in rare cases. Most frequently it is necessary that the living cells of the organism which receives the antibodies—antitoxin, fixatives or others—should contribute their quota in order to ensure the refractory condition. This rule is undoubtedly applicable to the examples of immunity acquired by the new-born progeny of unaffected mothers.