152 THE PRINCIPLES OF IMMUNOLOGY 



was the first to interpret the phenomenon in an immunological sense. 

 He pointed out that the penetration of bacteria into living cells, as 

 previously maintained by Birch-Hirschfeld, probably had much to do 

 with the disappearance of bacteria following injection. Subsequently, 

 it was shown that bacteria do not penetrate into cells but rather are 

 taken up by the cells. This work of Panum did not lead to any direct 

 result in the study of immunology, for Metchnikoff in his early work 

 on the subject was ignorant of it. Roser had also stated that according 

 to his opinion the immunity of healthy animals and plants against in- 

 fectious organisms rests upon (a) the relative salt content of their 

 fluids and (b) the capacity of their contractile cells to take up the 

 invading organisms. Roser, however, did not support this statement in 

 later studies, and again Metchnikoff was ignorant of this work when 

 he took up his great work on phagocytosis. Metchnikoff had studied 

 extensively the nutrition of certain of the lower forms of animal and 

 vegetable life and also their defenses against the invasion of harmful 

 parasites. From this work he was led to the conclusion that the defense 

 of higher animals depends in great part upon the phenomenon of 

 phagocytosis. This statement was magnified into a conflict between 

 the so-called cellular theories of immunity and the humoral theories 

 of immunity. At the present time, however, such a conflict does not 

 exist because the two theories of immunity are in perfect harmony with 

 one another, and it is known that they are dependently interrelated. 



The process of phagocytosis involves three steps; first, the ap- 

 proach of the cell and the material to be taken up ; second, the ingestion 

 of the material, and, third, the destruction of such material as may be 

 dissolved by the digestive fluids of the cell. The problem of the 

 approach of the cell and material to be ingested is one fundamentally 

 of irritability. The irritability of living tissue is in response to certain 

 stimuli and such stimuli include chemical, thermal, osmotic, photic, 

 mechanical and other physical agents. In the early studies of the 

 physiology of stimulation the response of a cell to a stimulus was 

 believed to be governed by the Weber-Fechner law, which states that 

 the intensity of sensation varies with the logarithm of the intensity 

 of the stimulus or, in other words, as the stimulus increases by geometric 

 progression the response increases by arithmetical progression. This 

 law has been found by further study to be untenable, for it has been 

 shown that logarithmic functions are not applicable to very strong 

 stimuli. In phagocytosis chemical stimuli are the most important, and 

 the response to such stimuli is referred to as chemotaxis. 



Mutual Approach (Chemotaxis). Chemotaxis may be positive or 

 negative, according to whether it attracts the two bodies or repels them. 

 Such attraction or repulsion does not depend essentially on the acidity 

 or alkalinity of the medium but does depend in certain measure upon 

 its concentration. Not only does variability of concentration play a 

 part, but the adaptability of the cell itself is of importance ; for example, 

 myxomycetes plasmodia exhibit negative chemotaxis in the presence 

 of sugar in certain concentrations, but after the organism becomes 



