IMMUNITY AND SUSCEPTIBILITY. 567 



there is no phagocytosis and immunity is due to antibacterial substances in 

 the blood. 



Hemoopsonins. It has been demonstrated that very frequently 

 opsonins for red corpuscles are present in the serum and body fluids of ani- 

 mals. Such bodies sensitize the red blood corpuscles and render them 

 susceptible to phagocytosis by the polymorphonuclear leucocytes and the 

 epithelial and other body cells. They are designated as hemoopsonins. 

 Occasionally iso- and autohemoopsonins are present in normal sera. 

 For example, in human serum, it is probable that the processes of red 

 blood corpuscle destruction which take place in the spleen may be re- 

 ferred to the action of these types of opsonins and various phagocytic 

 cells. 



AGGLUTININS. Agglutinins are substances which are present in the 

 blood serum and body fluids of normal and immune animals which have 

 the power of producing a clumping and sedimentation of the microorgan- 

 isms causing the specific infection or used in immunization. The rela- 

 tionship of the agglutinins to the phenomena of immunity and the other 

 antibodies which are produced during the process of infection and ex- 

 perimental inoculation is not known. One of the first agglutinins to be 

 observed was that occurring in the blood serum in typhoid fever and the 

 agglutination reaction is now made use of in the diagnosis of this disease 

 (Widal test). Agglutinins are specific substances and at high dilutions 

 only cause a clumping of the microorganisms which give rise to their 

 formation (antigens). 



Normal Agglutinins. Agglutinating substances, as above stated, are 

 frequently found in normal sera. In this case no direct connection between 

 their formation and specific microorganisms can be established. Normal 

 human serum frequently contains agglutinins for B. typhosus, B. coli, Bad. 

 dysenteric and occasionally M. pyogenes aureus and Msp. comma in certain 

 rare cases. Agglutinins for B. typhosus which are present normally in the 

 serum may give rise to confusion when this test is used for the diagnosis 

 of typhoid fever. It is therefore necessary to dilute the serum of a sus- 

 pected case of typhoid fever at least one to forty or one to fifty times in order 

 to exclude the normal agglutinins and the so-called coagglutinins. 



The Production of Agglutinins. Agglutinins may be produced arti- 

 ficially by the injection of bacteria, dead or alive, into the veins, subcu- 

 taneous tissues, or peritoneal cavity. In exceptional cases they may be pro- 

 duced by feeding the bacteria, injecting them into the air passages of the 



