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 assimilative powers of the vegetable 

 kingdom, and have to supply their needs by using the members of this king- 

 dom 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 microsphera 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 tho 

 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 those belonging to the natural 

 order Leguminosae. In herbivorous animals the presence of specific bacteria 

 in the paunch or caecum causes the breakdown of the cellulose walls of the 

 food and may indeed lead to a building up of protein from amino- acids or 



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