Antibodies and Immunity 87 



killed or damaged bacteria or viruses (vaccines) sometimes conferred 

 immunity against infection by living bacteria. In making prepara- 

 tions of this kind it is necessary to be sure that all the bacteria are de- 

 stroyed, to the extent that they are incapable of multiplication; but at 

 the same time their ability to cause immunity must be retained. This 

 is obviously a rather tricky business and instead of killing the bac- 

 teria by heating, treatment with chemicals such as formalin is used 

 in the preparation of vaccines. For example a successful vaccine 

 against poliomyelitis has been produced by treatment of the virus 

 with formalin. 



From innumerable experiments of this kind it was concluded that 

 the body responds to the introduction of foreign organisms by pro- 

 ducing substances called antibodies, which are able in some way to 

 neutralize or destroy the foreign bodies. The course of a typical in- 

 fectious disease is (1) infection, (2) multiplication of the infective 

 agent, (3) antibody formation which, if it is extensive enough, may 

 lead to the neutraUzation and destruction of the agent. 



In some cases once formed, the antibodies or the ability to make 

 them may remain in the body of the recovered person for consider- 

 able periods and even for all the rest of his life and, in that case, he 

 will probably be immune from the disease if he receives another in- 

 fection. However, in other diseases, such as influenza, immunity is 

 only conferred for a limited period, i.e. the antibodies formed are lost 

 or become ineffective. 



It is obvious that in this reaction animals have developed a very 

 effective means of destroying invading micro-organisms. A great deal 

 of research has been done to discover its mechanism and although 

 much still remains to be learnt about it, the main features are now 

 known. 



Antibodies are formed in the animal organism when a foreign pro- 

 tein (i.e. a protein from another species) or certain other substances 

 are injected into an animal. These have the power of combining with 

 the foreign substance and in many cases of causing it to be preci- 

 pitated. Substances which are capable of invoking the production of 

 antibodies are called antigens. They include not only proteins (and 

 some proteins are better antigens than others) but also complex 

 carbohydrates. The reaction between an antigen and its antibody is 

 highly specific and also quantitative, i.e. a quantity of antibody will 

 combine with a definite quantity of its own antigen, but not with 

 others. 



Bacteria, of course, contain many substances which could act as 

 antigens, but the most effective ones are probably the substances of 

 the cell walls. When antibodies to these are formed they combine 



