TYPES OF IMMUNITY 147 



Introduction of the Dead Causal Agent. This method is of 

 course safer than the preceding one, and the immunity produced 

 is identical with that produced by the injection of living organisms 

 save that it is of a lower degree and is not so lasting. 



Vaccines usually produce a general reaction, such as malaise, 

 headache, pains in the muscles, and slight fever, and a local reac- 

 tion at the point of inoculation. The reactions appear as a rule 

 within a few hours and last from one to two days. 



A vaccine of dead organisms is prepared from a twenty-four- 

 hour growth of a pure culture on an agar slant. The growth 

 is washed off with salt solution, and after the number of organisms 

 present in the suspension are determined they are killed by being 

 exposed to a temperature of 56 C. for one hour. A higher temper- 

 ature for a shorter period would be equally effective in killing the 

 bacteria, but it would at the same time so alter the organisms 

 chemically as to make them less effective. 



Several methods are in use for determining the number of bac- 

 teria present in such a suspension ; the following is one of the most 

 frequently employed. Blood is taken from a pricked finger and 

 a blood count made to ascertain the number of red blood corpuscles 

 present in a cubic millimeter. Then with a capillary pipette 

 one volume of blood is taken from the pricked finger and mixed 

 with one volume of the bacterial suspension and two or three 

 volumes of sterile salt solution. The mixture is then spread 

 evenly on a slide as in making a blood smear, and after staining 

 with one of the special blood stains it is examined with the oil 

 immersion lens. The red blood cells and the bacteria are counted 

 in a certain number of fields and the ratio between them deter- 

 mined. If, for example, in twenty fields the average shows two 

 red blood cells to one microorganism and there are five million 

 red blood cells in each cubic millimeter of blood, then there will be 

 approximately half that number of bacteria, or two million five 

 hundred thousand, in a cubic millimeter of the suspension and in 

 a cubic centimeter one thousand times more. A vaccine con- 

 taining any number of bacteria desired can thus be obtained by 

 diluting the original suspension. A simpler method is that of 



