In agreement with Goldmann, Jadassohn and H. F. Müller, we believe that the only admissible explanation for the facts mentioned above is that the eosinophil cells obey specific chemiotactic stimuli. By this hypothesis we can easily understand eosinophil leucocytosis, the presence of eosinophil cells in exudations and secretions, and the local accumulation of this kind of cell.
As to the nature of these chemiotactically active substances, we can so far only surmise. From amongst the clinical phenomena capable of throwing light on this subject we mention once more the fact, that the metabolic products of bacteria repel the eosinophil cells.
The opposed behaviour of eosinophil and neutrophil cells is very well illustrated by a case of Leichtenstern:
"In a very anæmic almost moribund patient with Ankylostomias there were found 72% eosinophil cells in the blood in 1897. The patient contracted a croupous pneumonia, and in the high febrile period of the disease the number of eosinophils sank to 6-7%, and rose again after the termination of the pneumonia to 54%. After removal of the worm the number at once fell to 11%. In the year 1898 the patient harboured but a very few Ankylostomata; Charcot's crystals were no longer present in the fæces; the number of the eosinophils amounted to 8%."
The question, what cells produce on their destruction actively chemiotactic substances, is of very great importance; but cannot be answered with the material at present available. The breaking up of ordinary pus cells or lymphocytes does not appear to give rise to any such substances; but there is much evidence that the decomposition products of epithelial and epithelioid cells act chemiotactically. Thus we can explain the frequent occurrence of eosinophilia in all kinds of skin-diseases. Again, in all atrophic conditions of the gastric, intestinal and bronchial mucous membrane there occurs a local accumulation of eosinophil cells; further, this kind of cell is increased in the neighbourhood of carcinoma. Additional support for this view is seen in the fact that in bronchitis and asthma the less the suppurative element of the secretion is developed, the more numerous are the eosinophil cells. An observation of Jadassohn is worthy of mention in this connection. He observed abundant eosinophil cells in foci of lupus after injection of tuberculin. In these foci then, by the destruction of the epithelioid cells brought about by the tuberculin, substances must have been produced which act chemiotactically on the eosinophil cells.
The specific substances are absorbed and reach the blood, and impart to it also the chemiotactic power. The direct cause then of most forms of eosinophilia seems actually to lie in a destruction of tissue, and in the products thus produced.
On the other hand, it cannot be doubted that substances foreign to the organism, circulating in the body, may act chemiotactically on the eosinophil cells[32]. The observations quoted above, of the well-marked eosinophilia in the different forms of Helminthiasis, may here be specially mentioned. The action of the Helminthides was formerly regarded as purely local, but the indications that they act also by the production of poisonous substances continue to increase. Thus Linstow has pointed out that the general typhoid state, and the fatty degeneration of liver and kidneys, that is of organs which the Trichina does not reach, necessitate the assumption of a poisonous substance. And in several varieties of Ankylostoma as well, there is distinct evidence of the production of a poison. We gather from Husemann's article on "animal poisons" (Eulenberg's Realenencyclopœdie 1867) that just as Ankylostomum in man produces the well-known severe anæmia, so Ankylostomum trigonocephalum in the dog, and Ankylostomum perniciosum in the tiger, causes analogous general effects.
Bothriocephalus latus too is now generally accredited with the production of a definite toxic substance; and the common tapeworm even, by no means infrequently brings about injuries to the body which are to be referred to the action of a poison.
So much follows from these observations, that the tapeworms can not only absorb but also can give out substances that are absorbed from the intestine of the host, and are able to bring about distant effects. One expression of these distant actions is, as Leichtenstern insists, the eosinophilia of the blood. We do not think we should assume on the evidence before us, that the substance which attracts the eosinophil cells is identical with the cause of the anæmia. Many observations, the absence, for example, of eosinophilia in Bothriocephalus anæmia (Schauman), render probable the existence of two different functions. In any case the substance causing the eosinophilia is more widely distributed than that to which the anæmic condition is due.