OPSONINS, AGGLUTININS, PRECIPITINS, LYSINS 137 



Serum may be obtained by pricking the finger or ear lobe with a 

 sharp-pointed instrument or needle and allowing the blood to pass 

 into a Wright's capillary tube, as already described. Whole blood 

 may be used, in which case one or two drops are placed on a slide 

 and dried and later brought into solution again by the addition 

 of salt solution. The use of serum from which the red blood cells 

 have been removed is preferable. 



When the serum has been obtained a 1 in 10 dilution is made 

 by mixing one part of serum with nine parts of normal salt solu- 

 tion; from this dilution higher ones are made. For example, 

 one part of the 1 in 10 dilution to one part of salt solution gives a 

 1 in 20 dilution ; one part of 1 in 10 dilution to two parts of salt 

 solution gives a 1 in 30 dilution ; etc. 



The culture with which the test is to be made should be either 

 a twelve to eighteen hour growth in broth of the specific organisms 

 or an emulsion of an eighteen hour agar slant culture in normal 

 salt solution. The latter is prepared as already described for 

 an opsonic index. 



In making the microscopic test the procedure is much the same 

 as for a hanging drop. A platinum loopful of the serum dilution 

 and an equal quantity of the bacterial suspension are thoroughly 

 mixed on a coverslip, and the coverslip is then inverted over a 

 concave glass slide the rim of which has been greased with vaseline. 

 Several dilutions are used and the slides are placed in the incubator 

 for twenty to thirty minutes and then examined with the number 7 

 dry lens. In a series of dilutions all degrees of agglutination may 

 be observed, from large clumps with clear interspaces to smaller 

 clumps with a few motionless bacteria between, down to clusters 

 of six or seven organisms partially agglutinated trying to free 

 themselves. In a positive case of typhoid fever, within thirty 

 minutes a 1 in 20 dilution will usually show complete agglutina- 

 tion, and a 1 in 40 dilution an almost complete reaction ; partial 

 agglutination may sometimes be observed in a dilution as high as 

 1 in 100. Properly killed bacteria respond to the test almost 

 as well as living ones. 



The macroscopic test is made as follows : a series of small tubes 



