274 AGGLUTININS 



If such a donor cannot be obtained, it is safer to use a person whose serum 

 is agglutinative toward the patient's cells than one whose cells are 

 agglutinated by the patient's serum. 



The technic of these transfusion tests is given at the end of this 

 chapter. 



Non-agglutinable Species of Bacteria. Certain species of bacteria, 

 especially when freshly isolated from the animal body, may prove them- 

 selves immune to the action of agglutinins; this is true especially of 

 the bacillus of Friedlander. As a rule, this resistance is lost when the 

 microorganism is grown for some time in artificial media. In some in- 

 stances the typhoid bacillus, when freshly isolated from a patient, may 

 resist agglutination until after it has passed a period of existence on 

 artificial media. This variability is probably due to some change 

 taking place in the agglutinable substance of the agglutinins during the 

 sojourn of the bacilli in the animal body, and possess such an excess of 

 agglutinogenic receptors as to require a much larger amount of aggluti- 

 nin to cause agglutination. 



It should be remembered that agglutinins act on dead as well as on 

 living bacteria, those killed by heat, formalin, phenol, etc., being simi- 

 larly agglutinable. In making the microscopic test the use of dead 

 bacteria is not so satisfactory as when the test is made with living motile 

 bacteria, for the influence of the serum on motility alone is of value in 

 interpreting a reaction. 



Variation in Agglutinating Strength of a Serum. In a given infection, 

 such as typhoid fever, there is usually a continued increase in the amount 

 of agglutinin in the blood from the fourth day until convalescence is 

 established, and then a decrease occurs. It is a fact of practical impor- 

 tance that the agglutinating power of a serum may vary from day to 

 day, so that it is very strong one day and may become weak or disap- 

 pear entirely on the next day or two. Hence the importance of making 

 more than one test in a suspicious case when the first trial has been 

 doubtful or negative. There is no satisfactory explanation for this 

 variation, mixed infection, intestinal hemorrhage, etc., being regarded 

 by some as responsible for it. 



Role of Agglutinins in Immunity. The agglutinins were formerly 

 regarded as possessing a true protective and curative power. It has 

 previously been mentioned that bacteria may be grown in a specific 

 agglutinating serum, and cultures made of agglutinated bacteria show 

 them to be fully alive and as virulent as before agglutination took place. 

 In certain cases agglutinins for a microorganism may be entirely absent, 



