j5^ LIFE : OUTLINES OF GENERAL BIOLOGY 



injection of vaccine microbes will enable the organism to witlistand 

 the intrusion of smallpox microbes. 



The other metliod is to inject a serum preparation of the blood 

 of an animal that has been immunised by graduated direct doses of 

 the poison, or by non-fatal attacks of the virulent microbes. Some 

 animal, such as a horse, is rendered immune, and its serum, abound- 

 ing in anti-toxins according to the theory, is injected into another 

 organism. This method is less drastic than the other, yet very 

 effective. It is often used as a protection against a probable poisoning, 

 whether by snake-bite or by some virulent microbe, as well as an 

 antidote after the introduction of the poison has taken place. It 

 might be thought that it would be better still, in the case of snake- 

 bite, to utilise the serum of a naturally immune animal, such as the 

 hedgehog or the mongoose. But there is only a small quantity of 

 the anti-toxin present in the blood of these naturally immune 

 animals; the counteractive effect does not last long; and there are 

 positive disadvantages in the use of the serum so obtained. 



Immunis.\tion by Disease. — Another form of immunity is very 

 familiar— namely, that which follows recovery from a disease. The' 

 individual is more or less protected against taking the disease a 

 second time. The immunity may last for many years in the case of 

 smallpox, but there are records of two or even three attacks; it may 

 be strong, as in the case of scarlet fever, measles, and mumps; it 

 may be transient, as in diphtheria. There are rare cases, such as 

 })neumonia: here the immunity is at most very temporary; where, 

 indeed, susceptibility to the disease appears to be increased, not 

 decreased. 



There is evidence in some instances that an artificially immunised 

 rabbit or guinea-pig has young ones which are born immune. The 

 same is asserted in regard to smallpox, that the immunised human 

 mother may confer immunity on her offspring; but it is difficult to 

 prove this satisfactorily. If it occurs, it may be due to specific 

 anti-toxins passing \ia the placenta from the blood of the mother. 

 When there is evidence of immunity as a racial character— as 

 in the case of negroes, who are relatively immune to yellow fever 

 and malaria, or in the case of Algerian sheep, which arc relatively 

 immune to splenic fever or anthrax- the explanation is probably 

 that a constitutional variation, like the hedgehog's, arising apart 

 from injection or other poisoning, has become hereditary, whereas 

 those members of the stock who did not vary in the direction of 

 immunity would be gradually eliminated. 



Field for Kese.akch.- For many years Ehrlich's "side-chain" 

 theory of immunity was greatly admired, and it has been provocative 

 of useful investigation. But it is no longer in favour; and many 

 authorities would not at present go farther than say that immunity 

 in animals, whether natural or acquired, implies a subtle peculiarity 



