IMMUNITY 83 



Wright (1902) showed that sterile cultures of certain micro- 

 cocci given in small doses raised the agglutinating power of 

 the blood to that particular organism. Wright (1903) using 

 Leishman's method, found that the phagocytic index is raised 

 by the injection of bacterial products. 



Wright and Douglas (1903) gave the name of opsonin (from 

 opsono, I prepare the food for) to this element in the blood, 

 arguing that in some way the bacteria are acted upon by this 

 substance, making them more readily digested. 



Opsonic Index. The degree of immunity of a given blood- 

 serum, as compared with that taken from a healthy individual, 

 considered as a unit, is called the index. 



The activity of the leukocytes toward an emulsion of bacteria 

 is noted first when washed with normal serum, and then when 

 washed with serum from an affected person. If, in the one 

 instance, in 50 cells an average of 3 is obtained, and, in the 

 other instance, the average is only ij, the index is then said 

 to be 0.5, or one-half of the normal. This index is measured 

 from day to day with the course of the disease and treatment, 

 and charted just as the temperature is. 



A negative phase, that is, a decreased opsonic index, often 

 follows the use of bacterial products; then comes a positive 

 phase, or a rise in phagocytosis. Injections of vaccines are 

 harmful if repeated before the negative phase has worn off. 



Summary of Immunity Theories. At this writing the 

 following ideas concerning immunity seem to be accepted: 



The blood-serum contains substances opsonins which 

 envelop or attach themselves to bacterial cells, rendering them 

 fit for leukocytic digestion phagocytosis. The growth of 

 bacteria in the body stimulates the production of antibodies, 

 antitoxins, and the activity of the blood-cells likewise calls 

 forth an increased activity of the germ and an increase in toxin. 

 The theory of Ehrlich, or chemical explanation of the action 

 of antitoxin, is still a subject of controversy. 



