174 THE VARIOUS TYPES OF IMMUNITY 



free state within the body-fluids, even though unbound to the body- 

 cells; this condition is not well understood, and may be dependent upon 

 other factors. A rest may restore the activity of the receptors and cells, 

 a fact that is well recognized in the immunization of horses for the prepara- 

 tion of antitoxin. Not infrequently a rabbit fails to produce hemolytic 

 amboceptor if the injections of erythrocytes are too frequent. After 

 a rest, however, the animal may react promptly with much smaller 

 doses. 



Passive Acquired Immunity. As the name indicates, this is a form 

 of immunity that depends upon defensive factors not originating in the 

 person or animal protected, but is passively acquired by the injection of 

 serum from one that has acquired an active immunity to the disease in 

 question. 



This is a sort of secondary immunity, acquired by virtue of having 

 received antibodies actively formed by another animal that has had to 

 resist the infecting agent in order to produce them. Two well-known 

 examples of this type of serums are the diphtheria and tetanus anti- 

 toxins. These are produced by injecting horses with successive doses 

 of the respective toxins. The horses are required to combat the effects 

 of the toxins, and acquire an active immunity of increasing grade, due to 

 the production of antitoxins. When the animals are bled, the antitoxin- 

 laden serum, separated from the corpuscular elements, may be used for 

 conferring an immunity in a person or another animal simply by injecting 

 the serum, the latter receiving and enjoying an immunity in a passive 

 manner. 



Passive immunity is specific, that is, the serum of an animal im- 

 munized against one microorganism will protect a second animal against 

 that and against no other. This type of immunity is acquired just as 

 soon as the immune serum has become mixed with the blood of the per- 

 son or animal injected, and there is no negative phase. Hence in 

 severe infections our hopes of specific therapy rest on the production of 

 passive immunity. No matter how sick the recipient may be, under 

 ordinary circumstances the immune serum produces no further dis- 

 turbance than would be expected from the injection of a normal serum. 

 The recipient's body-cells have no additional burdens, or very slight 

 ones only, to bear and these are more than counterbalanced by the re- 

 lease from combat with toxic substances neutralized by the antibodies 

 in the immune serum. Unfortunately, this field of therapy is limited, 

 although recent discoveries are indicating the reasons for failure, and 

 when these are eliminated, the field of usefulness will be much extended. 



