IMMUNITY 65 



causes the cessation of the motility of motile bac- 

 teria. This form of anti-body is known as "aggluti- 

 nin." The Widal test as a means of diagnosis of 

 typhoid fever is based upon this agglutination of 

 bacteria by blood-serum. A drop of blood from 

 the patient is added to a specially prepared culture 

 of typhoid bacilli; if the bacilU become motionless 

 and soon clump (agglutinate) in irregular masses, 

 the patient is doubtless infected with the bacillus 

 of typhoid. Other species of bacteria are also 

 agglutinated by their own anti-sera. It is not 

 thought that agglutinins are bactericidal. 



" Of the hypotheses ' advanced in explanation of 

 acquired immunity, the one worthy of greatest con- 

 fidence is that which assumes immunity to be due to 

 reactive changes on the part of the tissues that result 

 in the formation in these tissues of antitoxic and 

 other anti-bodies which circulate free in the blood 

 and in a variety of ways serve to protect the tissues 

 from the harmful effect of extraneous intoxicants 

 and irritants, in some cases acting principally as 

 antidotes to a toxin, in others exhibiting more the 

 germicidal (bacteriolytic) than the antitoxic prop- 

 erty." 



* Abbott, Principles of Bacteriology. 



