CHAPTER XV 



THE DEFENCE OF THE ORGANISM AGAINST 



INFECTION 



SECTION I 

 THE CELLULAR MECHANISMS OF DEFENCE 



ONE of the main distinctions, perhaps the most important, between 

 the animal and vegetable kingdoms lies in the inability of animals to 

 build up their tissues at the expense of inorganic salts, and especially 

 to synthetise the various groups necessary for the formation of the 

 protein molecule. They are thus rendered dependent on the assimila- 

 tive powers of the vegetable kingdom, and have to supply their 

 needs by using the members of this kingdom as food. The protozoa, 

 for example, subsist largely on bacteria. To obtain a pure culture 

 of any form of amoeba it is necessary to cultivate this along with some 

 form of bacteria. The power of the unicellular animals to digest 

 bacteria meets with a response on the part of the latter, many of them 

 developing, by way of self-defence, the habit of forming and excreting 

 poisons which will deter the amoeba from taking them up or injure it 

 after it has ingested them. There is thus a continuous struggle among 

 the various grades of unicellular organisms in which sometimes one, 

 sometimes another type survives. An amoeba placed in contact 

 with most kinds of bacteria, living or dead, will rapidly englobe and 

 digest them. There is, however, a small organism known as micro- 

 sphcra which is taken up by the amoeba, but is not thereby destroyed. 

 Retaining its vitality, it reproduces itself rapidly in the body of its 

 host and finally leads to disintegration of the latter. In the same 

 way the flagellate protozoa are often infected by a species of fungus 

 known as chytridium, and die in consequence. 



The liability of organisms to infection by others endeavouring 

 to live a parasitic existence at their expense extends throughout the 

 whole of the animal and vegetable kingdoms. In some cases the 

 host and the parasite arrive at a compromise in which each benefits 

 the other. This condition is known as symbiosis. We have examples 

 of it in the union of fungi and algae which occurs in lichens ; in the 

 association of nitrogen-fixing bacteria with many plants, especially 



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