BIOLOGICAL PRODUCTS (VETERINARY). 
6G9 
natural conditions or under those experimentally produced. Re¬ 
sistance to infection is often a natural inborn quality of a species 
or an individual. Such resistance is named natural immunity. 
Immunity may be acquired by an animal either by passing 
through an attack of the disease, or by means of artificial inocu¬ 
lation. It is with this latter method of producing immunity that 
the veterinarian is concerned. According to the process by which 
it is produced, acquired immunity may be said to be of two kinds, 
active and passive, the former being more or less permanent and 
the latter temporary. 
Active Immunity. Active immunity may be obtained by one 
or more injections of the organisms, in a weakened condition or 
by sub-lethal doses of virulent organisms or their toxins. By 
increased doses at varying intervals, the animal will develop in 
the course of time a comparatively high degree of resistance to 
the material injected. Such means constitute the process of pre¬ 
ventive inoculation or vaccination. The production of active im¬ 
munity is a comparatively slow process. The resultant resistance 
lasts a considerable time, the duration varying in different cases. 
Passive Immunity. Passive immunity depends upon the fact 
that if an animal has been rendered immune by any of the meth¬ 
ods used in the production of active immunity, its serum will 
contain substances which have distinctly antagonistic or neutraliz¬ 
ing effects on the specific bacteria or their toxins, whichever the 
case may be. Such sera may be used prophylactically or thera¬ 
peutically in the early stages of the disease. 
In the case of active immunity the animal protects itself 
through its own activities, the cells of the body participating in 
the process. In passive immunity, protection depends upon sub¬ 
stances developed in some other animal, the individual receiving 
these substances having practically no part in the immunizing 
process. 
Antibodies. Among the anti-substances, or antibodies devel¬ 
oped in the tissues of an infected animal, or one actively im¬ 
munized may be named antitoxins, lysins (bacteriolysins), 
opsonins, agglutinins and preeipitins. 
