IMMUNITY 195 



part in intestinal digestion, thereby contributing to the func- 

 tioning activities of the host while thriving on the products of 

 digestion in the intestine. Parasites, finally, live at the expense 

 of their hosts. These are all illustrations of physiological adap- 

 tations on the part of symbionts, commensals, or parasites, 

 but adaptations do not stop here. 



C. ADAPTATIONS AGAINST PARASITES 



Parasites, especially some forms of bacteria, give off prod- 

 ucts in the form of nucleo-proteins or other chemical com- 

 pounds, which act as poisons on the host's cells and tissues. 

 Sometimes, as in typhoid fever or cholera, these poisons from 

 the intestine are absorbed into the vascular system and 

 are carried to all parts of the organism. The action of such 

 poisons differs in different cases. Very frequently the proto- 

 plasm and walls of the cells of the digestive tract are dis- 

 solved, a phenomenon known as lysis (hence cytolysis, hemo- 

 lysis, karyolysis, etc.) and local or distributed areas of func- 

 tioning organs are ulcerated and destroyed, leading to various 

 forms of enteritis or intestinal inflammation. Or the parasites 

 may become localized in other parts of the body, pneumococcus 

 in the lungs giving acute congestion characteristic of pneu- 

 monia; or the bacillus of tuberculosis in the lungs, which forms a 

 poison causing destruction of the lung cells as in consumption. 

 The organisms of tonsilitis and diphtheria accumulate in the 

 throat and give off poisons which act on the entire system; 

 the organisms of smallpox and scarlet fever collect in the skin, 

 those of hydrophobia in the nerve cells of the peripheral and 

 central nervous system, while some find their way into the blood 

 and multiply there; for example, Streptococcus and Staphylo- 

 coccus cause blood poisoning or malarial organisms cause 

 malaria. 



These various parasites have become adapted to this parasitic 

 mode of life in the human organism. They live at the expense 

 of the latter, and in the course of their various metabolic ac- 

 tivities they give off substances which interfere with the nor- 

 mal activities of metabolism of the host, usually by the direct 



