660 PATHOGENIC MICROORGANISMS 



One attack of typhoid fever protects against subsequent infection. 

 Accurate statistics upon the -matter have been difficult to obtain, 

 however, because histories of the disease are apt to be indefinite, and 

 until recently, no proper differentiation was made between true typhoid 

 fever and the paratyphoid group. However, taking into consideration 

 these possibilities of error, the estimations made by various clinicians 

 who have studied the subject, indicate that a second attack of typhoid 

 fever occurs in not more than from 0.7 to 4 per cent of all cases. Two 

 to 3 per cent represents a fair average of all estimates made. When 

 typhoid fever does occur for the second time, it is usually of a milder 

 type than the first attack, though, according to V. Vaughan, Jr., this 

 is not always the case. 



Circulating antibodies disappear from the typhoid convalescent 

 usually within the first seven months after recovery. Permanent 

 immunity cannot, therefore, be explained upon the basis of serum anti- 

 bodies. The ultimate cause for permanent immunity, in all diseases 

 in which it occurs, must be regarded as depending upon the physiological 

 unit, namely, the tissue cell. It is likely that individuals who have 

 passed through an infection of this nature, thereafter retain a capacity 

 to react more rapidly and effectively to small quantities of introduced 

 antigen. A case in point is the well-known experiment of Wassermann, 

 who immunized a number of rabbits to typhoid bacilli until a highly 

 agglutinin titer was produced. He kept these rabbits until their blood 

 had returned to normal and no agglutinins could be found. Subse- 

 quently he reinoculated them with typhoid bacilli, at the same time 

 giving a number of normal control rabbits similar injections. The 

 previously treated rabbits responded with a rapid and powerful anti- 

 body production in contrast to the slower antibody curve of those that 

 had received the typhoid antigen for the first time. Recent observa- 

 tions by Moon on revaccination of previously vaccinated people, have 

 given analogous results; and it appears from this that a person, once 

 immunized, is capable of reacting with much greater promptness than 

 a normal individual. Our own idea would be somewhat as follows: 



When the typhoid bacillus enters the bowel of the infected subject 

 it begins to proliferate and gradually enters into the lymphatic system. 

 As a consequence, a small amount of antigen is gradually introduced 

 into the circulation, reaches the cells and stimulates antibody produc- 

 tion. In the normal individual this reaction is a slow one, and the 

 typhoid bacilli multiply with a speed disproportionate to the appearance 

 of antibodies. In the previously immunized individual or in the person 

 who has had the disease, the first absorption of small amounts of antigen 



