IMMUNOLOGY 



CHAPTER I 



INFECTION AND INFECTIOUS AGENTS 



Infection. — The phenomenon of infection is usually considered 

 one in which an infectious agent gains entrance to the tissues of 

 a susceptible organism (the host) and finds suitable conditions 

 for growth and development in its new environment. 



Classification of Parasites. — These agents show wide variation 

 in their habitat, size, shape, chemical composition, and physiological 

 functions. They are usually classified into groups such as (1) 

 animal parasites, (2) pathogenic fungi, (3) pathogenic bacteria, 

 and (4) viruses. These groups are not sharply marked off from 

 each other, but each embraces many organisms of doubtful 

 taxonomic standing. 



Kendall (1923) divides tlie disease-producing bacteria into two 

 main groups and discusses them under two headings: (1) the 

 life cycle of parasitic bacteria; (2) the life cycle of pathogenic 

 bacteria. The paradtic bacteria in contrast with the pathogenic 

 are deficient in invasive power and in the ability to escape from 

 the tissues. Both types of organisms are able to multiply within 

 the tissues of the host after they gain entrance. 



Goodpasture (1936) suggests that infectious agents be classified 

 as extracellular, facultative intracellular, and obligate intra- 

 cellular parasites. The immunologist was in reality first interested 

 in one of the virus diseases, smallpox, and many years later in 

 bacterial infections. ]\Iore recently lie has extended his interest to 

 infections caused by members of the first two groups. 



Symbiosis. — After infection has occurred, it may or may not 

 disturb the tissues of the host to any great extent, and in some 

 instances both parasite and host may be benefited. In the case 

 of root-nodule or nitrogen-fixing bacteria that produce tumorlike 

 gro^\1;hs on the roots of leguminous plants, the host is dependent 

 upon them for simple nitrogen compounds necessary for the life 



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