72 ANTIBODIES, IMMUNITY, WASSERMANN SERUM TEST 



injections have to be made at suitable intervals and a new one must 

 not be made before the effects of the previous one (prostration, fever, 

 etc.) have entirely disappeared. By this method there is manufac- 

 tured in the body of the animal, i. e., in its blood serum, an enormous 

 amount of antitoxin. After the proper prolonged treatment the blood 

 can be drawn off and the serum separated from the clot (everything 

 thing should be done, of course, under aseptic precautions). 



Antitoxic Units. The serum so obtained is called an antitoxin, 

 an antitoxic, or an immune serum. It is necessary to ascertain the 

 value of an antitoxic serum. This is done in animal experiments in 

 which both toxins and the antitoxic serum to be tested are used. The 

 animals employed in the experiments are generally guinea-pigs. 

 The unit measure of an antitoxin generally employed is that amount 

 which will protect a guinea-pig of about one-half pound weight 

 against ten times the ordinary fatal dose. For instance, it is found 

 that 0.01 c.c. of a strong diphtheria toxin will kill a medium-sized 

 guinea-pig. Then take 0.1 c.c. of the strong toxin and ascertain the 

 amount of antitoxin necessary to neutralize it. Suppose it is 0.0025 

 c.c. Then this 0.0025 c.c. of the antitoxic serum is said to contain 

 one immunizing unit, or 1 c.c. of this antitoxin contains 250 immuniz- 

 ing units. If in a case of diphtheria in a cat it is known from 

 experience that it will take 500 immunizing units to cure it, it will 

 be necessary to inject 2 c.c. of the antitoxic serum or use a serum four 

 times as strong and give 0.5 c.c. to get the same effect. In order to 

 protect a horse prophylactically against tetanus it is necessary to 

 give 10 c.c. to 20 c.c. of a strong tetanus antitoxin, which dose 

 contains several thousand immunizing units. 



IMMUNITY. 



Definition. Immunity may be defined as the ability of a higher 

 animal organism to resist invasion by, and multiplication of, patho- 

 genic microorganisms and to neutralize their poisonous products. 

 The agencies to which the protection is due have already been named 

 and explained. They are the phagocytic cells, the opsonins, anti- 

 toxins, agglutinins, lysins, precipitins, and a number of others. 



Congenital Immunity. Certain animals may possess a congenital 

 natural immunity. For instance, many warm-blooded mammals, 

 such as cattle, sheep, mice, rabbits, guinea-pigs, are susceptible to 

 anthrax infection, while adult dogs and rats possess quite a strong, 

 though not absolute, natural immunity against this infection. The 

 horse and man are susceptible to glanders; cattle are immune. 

 Man is susceptible to typhoid bacillus and cholera spirillum infec- 

 tions, while all our domestic animals are immune, as far as natural 

 infection is concerned. 



Acquired Immunity. Persons who have one attack of the following 

 diseases are generally immune against a second attack, viz., measles, 



