518 GENERAL CONSIDERATIONS 



550. Toxins and Antitoxins.— The living of one organism in the 

 body of another may impose certain hardships upon the latter. In 

 addition to its own wastes the body of the host must eliminate the wastes 

 produced by the other organism. Of course, if the relationship is one 

 of symbiosis the elimination of these wastes does not impose a hardship 

 sufficient to counterbalance the advantage accruing from the relation- 

 ship, but in the case of a parasite this may be a serious strain upon the 

 host. Whenever these wastes act as a poison in the body of the host 

 they are included under the general term of toxins. This term, however, 

 also includes all other poisons which may be introduced into the body, 

 whatever their source. 



To any such poison, either elaborated within the body by some 

 parasitic organism or introduced into the body in any manner, the body 

 reacts by producing a substance which tends to neutralize the poison 

 and which is known as an antibody, or antitoxin. Such a substance is 

 produced by the body in response to the presence of any foreign chemical 

 substance and is part of the self-regulatory function of the body by 

 which it can adjust or defend itself. By neutralizing the toxins, the 

 antitoxins safeguard the body cells against injury and give time for the 

 body to eliminate the cause of the disturbance. Since the response to 

 each toxin is specific, a different antitoxin is produced for each one. 



551. How the Body Fights Disease. — One method by which the body 

 fights disease is, as has already been indicated, by the production of 

 antitoxins. Another way is through the activity of the white blood 

 corpuscles, or leucocytes. A leucocyte is an ameboid cell which shows 

 a tendency to take into its body other organisms and other materials 

 in the same fashion as an ameba takes in bits of food. Normal body 

 cells are not attacked by the leucocytes, but cells in the body which 

 become abnormal or which are injured, or foreign cells of any kind, are 

 taken up by them and destroyed. When thus taking in other cells 

 •they are termed phagocytes (literally, eaters of cells). Phagocytes are 

 active in the destruction of certain cells in the body w^hen the absorption 

 of tissue is desirable. For example, they play a part in the absorption 

 of the tail of a tadpole when it changes into a frog. When injury results 

 in the death and destruction of cells in the body, the phagocytes attack 

 the dead and injured cells and by destroying them and clearing away 

 cellular debris pave the way for normal regeneration and the return of a 

 healthful condition. They are also active whenever disease-producing 

 organisms enter the body. Attracted to the place of entrance of these 

 infective organisms, apparently in response to the unusual chemical 

 stimuli due to the invaders, the phagocytes ingest and destroy them. 

 If the number of invading organisms is not great, the phagocytes may in 

 this way safeguard the body against the onset of disease. If, however, 

 the invading organisms are so numerous at the point of infection that the 



