THEORIES OF IMMUNITY 29 



their antibodies while in others (typhoid) they continue to 

 live and multiply without losing any of their virulence. With 

 rare exceptions, antibacterial sera not only do not cure dis- 

 eases but are rather more harmful than curative (typhoid, 

 plague cholera, tuberculosis). Thus preventive vaccination, 

 however efficacious and durable in small-pox and anthrax, is 

 usually very precarious and transient in many other diseases. 



A series of facts have been discovered which without 

 touching infectious diseases directly have, from the theoretical 

 and practical point of view, given interesting results. Thus 

 it has been recognized (Bordet) that the serum or the blood 

 of an animal of another species provokes on injection the 

 formation of an antibody which can precipitate the serum 

 or dissolve the blood cells and can precipitate the serum of 

 the first animal and these researches, as well as the discovery 

 of bacterial precipitins, have led to the serum diagnosis of a 

 large number of infectious diseases, as well as to the reaction 

 called "complement fixation" (Bordet and Gengou, Wasser- 

 mann and others). 



In order to understand the biologic action of antigens in 

 general, it is necessary to consider at first the characters of 

 the reactions which are common to each of them and then 

 for each antigen to study: 



1. Its direct and immediate action on the blood and the 

 tissues; in other words, on the normal antibodies in the blood 

 and cells. 



2. Its delayed action, that is to say, its action on the 

 " antibodies in excess" which, after a variable incubation 

 period, will have appeared in the organism. 



3. The nature of its compounds with these " antibodies in 

 excess" as well as the reactions which these compounds may 

 call forth in the tissues and in the blood. 



It is very difficult to appreciate accurately the direct and 

 immediate action of antigens on the organism because we 

 have no idea of the quantity of actual antigen, that is of the 

 really active substance which is only part of a mixture and 

 which we cannot extract in pure state. 



For example, we do know that a cubic centimeter of a 

 broth culture of diphtheria bacilli contains two or three him- 



