IMMUNITY 191 



part in intestinal digestion thereby contributing to the function- 

 ing activities of the host while thriving on the products of diges- 

 tion in the intestine. Parasites live at the expense of their 

 hosts. 



C. ADAPTATIONS AGAINST PARASITES 



These are all illustrations of physiological adaptations on the 

 part of symbionts, commensals, or parasites, but adaptations 

 do not stop here. Parasites, especially some forms of bacteria, 

 give off waste products in the form of nucleo-proteids or other 

 chemical compounds 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 

 protoplasm and walls of the cells of the digestive tract are 

 dissolved, 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 pneumonia; 

 or the bacillus of tuberculosis in the lungs which forms a poison 

 causing destruction of the lung cells as in consumption. Simi- 

 larly with other disease-causing parasites the organisms of 

 tonsilitis and diphtheria accumulate in the throat and give off 

 poisons which act on the entire system the organisms of 

 small pox and scarlet fever collect in the skin, those of hydro- 

 phobia 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 Staphylococcus giving 

 blood poisoning or malarial organisms giving 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 

 activities they give off substances which interfere with the nor- 

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



