82 ESSENTIALS OF BACTERIOLOGY 



masses. A modified form of agglutination, in which long 

 strings of bacteria are formed, is known as the "thread" reac- 

 tion. 



Agglutinins are of different kinds, and they are not an indi- 

 cation of immunity. A serum may give a strong agglutinating 

 reaction, but have little or no bactericidal power. The power 

 varies from day to day it is strongest in the blood-serum, but 

 is found in other fluids of the body. 



Agglutinogen is the name given to the substance on the 

 bacteria, while agglutinin is the term restricted to the sub- 

 stance in the serum. 



Precipitins. Animals immunized to certain bacteria or to 

 albumins of different sorts form bodies which cause the blood- 

 serum to give a precipitate when added to cultures of these 

 organisms or fluids containing the specific albumin. The 

 phenomenon has found forensic application in the identifica- 

 tion of blood-stains. 



Opsonins. Much work has been done in the last few years 

 to harmonize the various theories on immunity, and a new one, 

 advanced by Wright and Douglas in 1905, seems to have much 

 in its favor. 



Briefly stated, the leukocytes have no power in themselves 

 to act upon bacteria, but derive this property from the blood- 

 serum. In normal blood-serum Wright maintains there is 

 a body which he calls "opsonin," which becomes fixed to the 

 bacterial cells and makes them subject to phagocytosis. This 

 power of the serum is greater the more immune the animal 

 furnishing it is. 



It can be increased by whatever increases immunity; in 

 other words, it is coincident with immunity. 



History. Denys and Leclij (1895) proved that the serum of 

 a vaccinated rabbit altered bacteria so as to permit their inges- 

 tion by leukocytes. 



Mennes (1897) showed that this modification resides in the 

 serum and not in the leukocytes. 



Leishman (1902) devised a method of estimating the phago- 

 cytic power of serums. 



