TYPES OF IMMUNITY 149 



by filtration of a broth culture. The process of immunization is 

 started by giving exceedingly small doses usually in conjunction 

 with antitoxin; afterwards the doses are gradually increased. 

 The method has been employed in the case of snake venom and a 

 high degree of immunity thus produced. 



Passive Acquired Immunity. As the name indicates, this 

 form of immunity is passively acquired by virtue of receiving 

 antibodies formed by the body cells of an animal that has had 

 to resist the infecting agent in order to produce them. Thus in 

 order that a child may become passively immune to diphtheria 

 an animal must first combat the disease ; horses are injected with 

 successive doses of toxin and are required to overcome its effect 

 and acquire an active immunity of a high grade due to the produc- 

 tion of antitoxin. The horse then is actively immune because it 

 has manufactured its own antibodies. When its antitoxin-laden 

 serum is injected into the child, the child becomes passively im- 

 mune ; protection being due not to the activity of its own body 

 cells but to those of the horse. 



Passive immunity is specific ; that is, the serum of an animal 

 immunized against one microorganism will protect an individual 

 or another animal against that and against no other. Immunity 

 of this type is gained just as soon as the immune serum has become 

 mixed with the blood of the person or animal injected. It is of 

 much shorter duration than active immunity and the degree is 

 seldom equal to that of the latter. It is, however, especially of 

 value as a prophylactic measure against an acute infection that 

 has a relatively short incubation period. 



Ordinarily the injection of immune serum as a therapeutic 

 measure causes very little disturbance to a patient and this little 

 is more than counterbalanced by the release of the body cells 

 from combat with the toxic substances which are overcome by the 

 antibodies injected. 



Passive immunity may be antitoxic or antibacterial, according 

 to the antigen employed for the production of the specific immune 

 serum. 



