88 Inside the Living Cell 



with the cell surface and render the bacterium open to attack by 

 scavenger cells in the blood called phagocytes. 



Bacteria also produce harmful proteins called toxins, which are 

 often responsible for some of the effects of the disease. These toxins 

 also elicit the formation of antibodies — called antitoxins. Antitoxins 

 can be prepared by inoculating a suitable animal such as the horse 

 with the toxin and they can be used to help a patient — especially in 

 the period before his own antibodies have come into action. 



Antibodies are proteins and it has been found that they are present 

 among the blood serum proteins in the fraction which is distinguished 

 as y-globulin. It is possible that this globulin is made up entirely of the 

 different antibodies which the body possesses. If a person has had 

 measles, his y-globuhn contains the measles antibody and in fact this 

 antibody can be concentrated from the y-globulin of the blood. This 

 y-globuUn has been prepared for human use from the stocks of blood 

 plasma which are kept for emergency uses and confers immunity 

 against measles. However, it is not possible to separate the measles 

 antibody from others present, and it may be that there are less de- 

 sirable substances with it so it is not used too freely. 



Much research has been carried out in efforts to discover the 

 mechanism of antibody formation. As the antibody is a protein and 

 is produced as a response to the presence in the blood of foreign 

 antigens, it is reasonable to suppose that the antigen is present when 

 its antibody is made. The site of the synthesis of antibody is known 

 to be certain cells of the lymphatic system which are present in 

 various parts of the body. The antibodies pass into the blood through 

 the lymphatic vessels. 



As I mentioned above, the relation between an antibody and its 

 antigen is highly specific. Much work has been done to discover the 

 limits of the specificity of the antigen-antibody reaction. An antigen 

 can be modified slightly by having small chemical groups added to 

 it. When antibodies were mixed with slightly changed antigens it was 

 found by Landsteiner that they were capable of detecting very small 

 modifications of the antigen, such as the addition of a small chemical 

 group. 



However, the exact way in which the antigen influences the syn- 

 thesis of the antibody protein is not known. It has been suggested 

 that as the antibodies are all proteins of a similar nature, they are 

 kept, as it were, in a semi-fabricated state, like suits of clothes in cer- 

 tain *ready-to-wear' tailoring shops, waiting for the final touches 

 which will ensure an exact fit, to be put on in the presence of the 

 antigen. It has also been thought that this modification of the anti- 

 bodies to suit particular antigens consists only in local modifications 



