60 IMMUNOLOGY 



bring about certain changes which usually result in a secondary 

 inflammation accompanied by mononuclear cell infiltration. Ac- 

 cording to Rivers (1933) if the virus does not act rapidly and 

 explosively and the host cells are capable of multiplication, the 

 primary effect of infections is stimulation leading to cellular hyper- 

 plasia. Following the latter there is usually necrosis and then a 

 secondary inflammation. Rivers states that the balance between 

 stimulative and destructive tendencies of the virus is the determin- 

 ing factor between predominance of hyperplasia and necrosis re- 

 spectively. He cites smallpox lesions as examples of stimulation 

 followed by necrosis and liquefaction of cells, i.e., there is first a 

 hyperplasia of cells followed by vesicle formation. As examples 

 showing an overgrowth of tissue as the prominent feature he cites 

 warts and certain tiunors. When the infected cell ''is incapable 

 of dividing and multiplying as is the case of nerve cells, then the 

 primary pathological changes arc necrobiosis and lysis of cells or 

 the appearance within the cellular elements of inclusion bodies." 

 This latter phenomenon is observed in rabies. 



Leucocyte Count. — It is. an interesting physiological fact that 

 in a normal individual the red cells, white cells, and the platelets 

 are maintained at a fairly constant level. Walters (1934) has 

 shown that exercise and rest affect the concentration of red cells in 

 the venous circulation. Doan (1927) and others have observed a 

 rhythmical variation in the number of wliite cells. Mueller has 

 shown that the vegetative nervous system exercises some control 

 over the number and distribution of white cells. It is also of 

 interest to note that differential counts indicate that there is to a 

 great extent a constant percentage of each type of white cell in the 

 peripheral circulation. Under normal conditions there are, on an 

 average, approximately 5,000,000 red cells and 7,500 white cells 

 per cubic millimeter of capillary blood. 



There are a few infectious diseases characterized by a leucopenia 

 (low white count). Among these diseases are measles, malaria, 

 influenza, typhoid fever, and tuberculosis. The differential count 

 of a normal blood shows that there are approximately 60 to 70 per 

 cent neutrophiles, 2 to 8 per cent monocytes, 25 to 30 per cent 

 lymphocytes (large, intermediate and small), 1 to 3 per cent eosino- 

 philcs, and from i/^ to 1 per cent basophiles. In the five infec- 



