There are other objections to the great importance that Löwit has given to leukopenia. _À priori_ it is quite incomprehensible that the various substances, which in the fundamental test-tube experiment are able to exercise a distinct chemiotactic influence on the leucocytes, should under other circumstances need the intervention of the products of decomposition of the white blood corpuscles. Moreover clinical experience speaks in general against Löwit's theory. For in infectious diseases hyperleucocytosis is very common; and a transient leukopenia is equally rare.
This contradiction to the experimental results obtained by Löwit is easily explained when one reflects how different from the natural processes of disease are the circumstances of experiment. In this case the animal is by intravenous injection flooded at once with the morbid substance, and a violent acute reaction of the vascular and blood systems is the natural consequence. In natural infection, insidious and increasing amounts of poison come quite gradually into play, and for this reason, perhaps, hypoleucocytosis in the normal course of infectious diseases is much rarer than in the brusque conditions of experiment.
Upon the clinical importance of leucocytosis, particularly for the infectious diseases and their various stages, an enormous mass of observations has accumulated. Selecting pneumonia as the best studied example, in the typical course of this disease the constant occurrence of leucocytosis is undisputed; the increase usually continues up to the crisis, and then gives place to a diminution of the leucocytes until a subnormal number is reached.
Of special importance are the observations on an absence of leucocytosis in particularly severe or lethally ending cases (Kikodse, Sadler, v. Jaksch, Tschistowitch, Türk and others).
In many other diseases as well, the observation has been made that hyperleucocytosis as a rule is only absent in specially severe, or in some way atypical cases. Several observers (Löwy and Richter, M. Hahn, Jacob), have been able to demonstrate experimentally for various infections, that artificial hyperleucocytosis influences the course of an artificial infection most favourably. The question, in what way does this process contribute to the protection of the body, is at the present time under discussion, and introduces the most difficult problems of biology.