Disease Transmission and Mans Resistance 315 



tination reactions with red blcK)d cells, including O-A-B blood 

 typing and Rh grouping, will be considered in detail later. 



Precipitins 



If disintegrated cells or any proteins in virtual solution are 

 employed as antigens, an antibody termed a precipitin is formed 

 by the body of the injected animal. This antibody when put in 

 contact with the colloidal antigen will result in the precipitation 

 of the dissolved cellular material. Such an antigen is a pre- 

 cipitinogen. 



One of the intriguing uses of the precipitin reaction is in 

 medico-legal work, especially in the identification of blood stains 

 on clothing or on weapons used in assault and murder cases. 

 The exact technic of performing these tests is left for advanced 

 textbooks in the field. 



Opsonins 



In Metchnikoff's cellular theory of immunity, discussed earlier 

 in this chapter, it was pointed out that before cells could be phago- 

 cytized (or phagocyted) they had to be prepared for this migra- 

 tion through the walls of leucocytes. The agent which prepares 

 organisms for such engulfment is the antibody opsonin, which 

 literally means "to prepare food for." Even in the apparent ab- 

 sence of antibody, white blood cells ingest a certain number of 

 organisms, but in the presence of opsonins which are formed as 

 the animal builds up immunity to the invaders, a marked increase 

 in phagocytic activity occurs. The difference between the normal 

 number of organisms ingested and the number taken in by leuco- 

 cytes in the presence of opsonin is used in calculating the so-called 

 opsonic index, which has some diagnostic significance in such 

 diseases as undulant fever and tularemia. 



Lysins 



These are antibodies which lyse (dissolve) cells. But in addi- 

 tion to these antibodies, a second non-specific thermo-labile (de- 



