70 AN INTRODUCTION TO ZOOLOGY 



be brought into contact with infected people. These last are said 

 to be immune to the disease, and those only slightly affected to be 

 partially immune. In a similar way, one kind of animal is quite 

 immune to diseases dangerous to others, for example, birds are not 

 susceptible to the particular malaria common to man. These are 

 examples of what may be termed natural immunity as opposed to 

 another variety, namely, acquired immunity. After an attack of 

 some diseases, like whooping-cough and chicken-pox, usually over- 

 come in childhood, the person is not liable to a second attack, because 

 immunity has been acquired as a result of the first attack. Immunity 

 may also be acquired in an indirect way, by means of vaccination, 

 as was first shown by Jenner. Thus in the case of small-pox, it has 

 been found that if a calf is inoculated with the disease, there is 

 produced in it a strain of germs whose virility has been very consider- 

 ably lowered. These when re-inoculated into a human being are 

 not strong enough to produce more than a temporary disturbance, 

 but yet are strong enough to produce a measure of immunity. Most 

 of the ill effects of these complaints are due to poisonous substances, 

 toxins, produced by the organism causing the disease. When im- 

 munity has been acquired, it is found that the blood contains sub- 

 stances capable of combining with the toxins and rendering them 

 inert and harmless, and these are termed the antibodies or antitoxins. 

 The available evidence seems to show that these bodies are produced 

 in the tissues and not in the blood, which only serves as a distributing 

 agent . They continue to be produced long after the original stimulus 

 has disappeared ; in some cases it may be for years. This immunity 

 by the production of antitoxins is not limited to the effects of disease 

 germs, for it applies also in the case of most snake poisons and some 

 plant poisons. 



Yet another kind of immunity is to be noted, and this is a most 

 important one in the case of men living under war conditions. In 

 this variety no antitoxin appears to be produced, but by the injection 

 of so many million bodies of dead bacteria the tissues and fluids of 

 the body produce a bacteriolytic substance or substances, whereby 

 they acquire the power of being able to at once attack and digest 

 the living bacteria, hence the disease can make no headway and the 

 invading germs are destroyed. 



The power of a man to withstand diseases is then dependent 

 to a very large extent on this ability to produce antitoxins and 

 bacteriolytic substances, and also on the readiness with which the 

 lymphocytes will exhibit phagocytosis. The ability varies in the 

 same individual from time to time, but may be tested by certain 

 reactions, and what is termed an opsonic index, or an approximate 

 measure of this capacity can be obtained. 



