172 INFECTION AND IMMUNITY. 



other hand, though the conditions be in general favorable, substances 

 may exist or be formed in the body which destroy the invading germs. 

 In other words these may at once or soon be disposed of by germicidal 

 substances, either in cells or in solution in the body fluids. 



Or, the toxic substances which micro-organisms set free, as in the 

 process of their nutrition they decompose organic ingredients of the tis- 

 sues or body fluids, may be rendered inert by further decomposition or 

 combination with substances present or formed in the tissue fluids. Or, 

 furthermore, the cells which are susceptible to the presence of the toxins 

 may become less vulnerable by adaptation to the deleterious effects of 

 the latter. Thus by a total unsuitability of the tissues or of the general 

 conditions to bacterial growth; by destruction of the invading germs; 

 by neutralization or destruction of toxins ; or, finally, by a capacity of 

 resistance or tolerance won through adaptation to a new and intrinsically 

 harmful environment, we may conceive of conditions w^hich in a measure 

 account for the known phenomena of immunity. 



Immunity from an infectious disease may be natural or hereditary. ' 



The absolute or relative insusceptibility of turtles and fishes for tet- 

 anus, of rats for anthrax, and of the lower animals for syphilis and for 

 scarlatina, are examples of hereditary or natural immunity which we 

 need not further consider here. Immunity is variable and rarely abso- 

 lute, and is subject to individual as well as racial variation. 



On the other hand, immunity may be acquired. Acquired immunity 

 may be secured by an attack of the disease from which the individual 

 has recovered natural immunization. Or, immunity may be acquired by 

 the introduction into the body of some material which gradually dimin- 

 ishes susceptibility without inducing distinct disease artificial im- 

 munization. Acquired immunity may be transmitted from parent to 

 offspring. 



Most of the infectious diseases appear to confer a certain degree of 

 insusceptibility to subsequent attacks of the same disease, though this 

 may be partial and temporary. But the exanthemata afford the most 

 striking examples of acquired immunity after an attack of infectious 

 disease. 



Let us now look more closely at some of the ways in which the body 

 may thus protect itself from the consequences of infection. 



It is well known that bacteria artificially introduced into the blood 

 of animals may, after a short time, wholly disappear from the circulat- 

 ing fluid, and be found in large numbers in leucocytes and other cells. 

 We have already seen in the study of inflammation (see p. 116) that cer- 

 tain cells of the body are capable not only of taking up micro-organisms 

 which enter the tissues, into their protoplasm, but may there kill and 

 destroy them (Fig. 86). This mode of destruction of micro-organisms, 

 largely by leucocytes, but also by other inesodermal cells, which when 



1 For a study of the theories of natural immunity see Midler, Cbl. f. Bak., Abth. I., 

 Bd. xxxiv., 1903, pp. 458, 550, 700. Fora study with bibliography of the protective 

 processes in natural immunity see Kisskalt, Zeits. f. Hyg. u. Infkr., Bd. Ixiv., 1903, p. 1. 



