PROTOZOA AND DISEASE 211 



Reactions of Hosts to Parasites in Disease. — In the present 

 connection we may consider some aspects of the host-parasite 

 relationship that have a bearing upon the problems of infectious 

 diseases. Similar relationships probably obtain in other cases cf 

 parasitism, although the knowledge of such conditions is mainly 

 derived from the study of disease in man and the domesticated 

 animals. One feature of general importance is that most para- 

 sites are specialists. One might think that a given species of 

 parasite would invade many hosts. This is the case in some 

 instances; but on the whole the parasitic species are remarkably 

 limited in their range of habitat, since they are able to survive 

 only in a single species of host or a few closely related species. 



Many interesting reactions might be considered under the head 

 of adjustments by the host to its parasites. The simplest of these 

 is mechanical -protection by the fonnation of barriers against fur- 

 ther invasion or injury, as when cysts are developed by the con- 

 nective tissue of a host in a way that walls off the parasites. 

 Another important means of defense is that known as phagocytosis. 

 The white blood cells, or phagocytes, and, to a lesser extent, other 

 cells such as those of the mucous membrane and connective tissue, 

 regularly ingest and destroy invading organisms like bacteria, 

 as does the amoeba with its prey or the endodenn of a hydra 

 during intracellular digestion {cf. p. 261). This relationship is so 

 well adjusted in the higher animals that the number of white blood 

 cells increases rapidly in response to infection by certain disease- 

 producing organisms. 



A more complex reaction is seen in what is termed immunity. 

 It is a familiar fact that a patient who has recovered from typhoid 

 fever, or certain other diseases, is immune to a subsequent attack 

 of the same disease. In some manner the individual is not as he 

 was before, because he has almost no chance of ever contracting 

 disease again. Something, which we call immunity, has been 

 acquired by his body. Apparently what has happened is a chem- 

 ical change in the plasma of the blood and IjTnph, but the exact 

 nature of this change is unknown. 



That the relationships are comphcated and far-reaching will 

 appear from the most general enumeration of the types of immunity 

 that are now recognized. Thus, immunity acquired by success- 

 fully withstanding the course of a disease is called natural immunity, 

 in contrast to the artificial immunity acquired through artificial 



