PNEUMOCOCCUS 167 



the death of a mouse or rabbit within twenty-four to forty-eight 

 hours from septicemia. After death the blood will be found to 

 contain enormous numbers of organisms. 



Little is known of the toxic substances produced by the pneumo- 

 coccus; it is thought that they are of the nature of endotoxins 

 and are closely bound to the cell substance. 



In characteristic pneumonia in man the organisms are found 

 in the bronchioles and alveoli of the infected lung and in the 

 lymphatic channels and blood capillaries; from the capillaries 

 they find their way into the general blood current. So abundant 

 are they in a certain percentage of cases that they may be found 

 in cultures made from 5 to 10 c.c. of blood. 



In all cases of lobar pneumonia and in many cases of broncho- 

 pneumonia pleurisy occurs, caused by the same organism that 

 gave rise to the pneumonia ; recovery from pleurisy due to pneu- 

 mococci is generally more speedy than that caused by streptococci 

 or staphylococci. 



Other infections frequently complicating pneumonia are those 

 of the pericardium, endocardium, meninges, and middle ear. 

 They are probably explained by the fact that the infecting organ- 

 isms are conveyed by means of the blood and lymph to all parts 

 of the body. 



Immunity. Following an attack of pneumonia immunity 

 lasts only for a short time. Two or three attacks of pneumonia 

 are not unusual for the same individual. A serum of some pro- 

 tective and curative value has been produced by successive 

 injections of gradually increasing doses of virulent pneumococci 

 into horses. In addition, such serum possesses specific agglutinins 

 which not only are an aid in diagnosis but by means of which in- 

 vestigators have been able to divide the pneumococci into four 

 groups according to their specific reaction. Group I is the cause 

 of the greatest number of infections. An immune serum has 

 already been produced which has met with considerable success 

 in conferring passive immunity in infections with this type. Fewer 

 cases are caused by Group II and Group III, but the mortality 

 is much higher than with Group I. A serum has been prepared 



