An explanation of the almost constant increase of the leucocytes of the blood in bacterial diseases is given by the chemiotactic as well as by the phagocytic theory of leucocytosis. The principle of =chemiotaxis= discovered by Pfeffer asserts that bacteria, or rather their metabolic products, are able to attract by chemical stimulus the cells stored up in the blood-forming organs ("positive chemiotaxis"). In the cases in which a diminution of the leucocytes in the blood is found, it is the result of a repulsion of the leucocytes by the bodies mentioned, =negative chemiotaxis=.
As the experimental investigation of leucocytosis was carried further, it was found that leucocytosis, quite similar to that occurring in infectious diseases, could also be brought about by the injection of various chemical substances (bacterio-proteins, albumoses, organic extracts and so forth); and it became evident that the explanation of the process by chemiotaxis must be supplemented in many respects. Löwit for instance found that when substances of this kind are injected, two different stages can be distinguished in the behaviour of the leucocytes. First came a stage in which they were diminished ("leukopenia," Löwit) and in such a way that only the polynuclear cells were concerned in the diminution, whilst the number of the lymphocytes was unchanged. After this came the phase of increase of the white blood corpuscles; and here too exclusively of the polynuclear cells; the =polynuclear leucocytosis=. This behaviour seemed to indicate that during the first period a destruction of white blood corpuscles brought about by the foreign substances took place, and that it was only the dissolved products of the latter which caused the emigration of fresh leucocytes by chemiotaxis. But new objections were raised against this view. Goldscheider and Jacob, in particular, shewed by exact experiments that the transient leukopenia of the blood was not true but merely apparent; and was caused by an altered distribution of the white blood corpuscles within the vascular system. For whilst in the peripheral vessels from which the blood for investigation was usually obtained, there was in fact a diminution of the leucocytes, "=hypoleucocytosis=," in the capillaries of the internal organs, especially of the lungs, a marked increase of the leucocytes, "=hyperleucocytosis=," was found.