8o ESSENTIALS OF BACTERIOLOGY 



humeral theory. " At present, modifications of Metchnikoff's 

 phagocytic theory and Ehrlich's side-chain theory seem the 

 most plausible. 



Phagocytic or Cellular Theory. Metchnikoff elaborated this 

 after his study on inflammation. Phagocytosis occurs in 

 animals when subjected to the action of an irritant. The leu- 

 kocytes are attracted to the injured spot and envelop the irri- 

 tating substance, be it bacteria or dead matter. The theory 

 given out at first was that if the leukocytes conquer the bacteria, 

 immunity results; if the bacteria eat up the leukocytes, disease 

 occurs. Modified to suit other conditions, as, for instance, 

 the germicidal properties of serum freed from its cellular ele- 

 ments, Metchnikoff states that at times phagolysis that is, 

 breaking up or solution of the phagocytes takes place, and 

 the fluids in which these cells are dissolved become charged 

 with the powers originally present in the phagocytes. Chemo- 

 taxis is the term applied to the attraction of bacteria for the 

 leukocytes, and is supposed to be chemical in its nature. The 

 phagocytic cells comprise: (a) The polymorphonuclear leu- 

 kocytes of the blood, termed microphages, and (6) a group 

 called macrophages, which includes all other cells having 

 phagocytic properties, such as leukocytes other than the poly- 

 morphonuclears, endothelial cells, and connective-tissue cor- 

 puscles. When these cells are injured, they set free their diges- 

 tive ferments, known as microcytases and macrocytases respect- 

 ively, which correspond to the alexins of Ehrlich. 



Ehrlich's Side-chain Theory. This derives its name from the 

 fact that it presents an analogy to what happens in the benzol 

 ring of organic chemistry when its replaceable atoms of hydro- 

 gen are substituted by "side chains" of more or less complex 

 nature. The molecule of protoplasm is supposed to consist of 

 a central atom group provided with a large number of side 

 chains which subserve the vital processes of the molecule by 

 combining with other organic molecules. These side chains 

 are called receptors, and are of many different kinds, so as to fit 

 them for combination with many different varieties of extrane- 

 ous groups. Bacterial toxins contain two groups: (i) The 



