Antibodies and Immunity 91 



keep skin banks for use in accidents. But it has been found, both with 

 human beings and animals, that although such grafts appear to be 

 quite happy for a few days, after this some reaction takes place and 

 the foreign skin is sloughed off. When a graft of a foreign skin is 

 tried a second time the wound rejects it from the start. 



This is an immune reaction. The body reacts to the skin of 

 another individual of the same species by producing antibodies (a 

 process which takes a few days) and these react with the grafted skin 

 and cause it to shrivel up. When the antibodies have already been 

 formed in a previous grafting operation this reaction is immediate. 



This is an even more highly specific response than the antibody 

 response to foreign proteins which was discussed above, as it dis- 

 tinguishes between the different individual members of one species. 

 Practically the only exception is the case of identical twins, which are 

 able to exchange skin grafts whether they are human beings or 

 cattle. In fact this provides a conclusive test (nearly 100 per cent true) 

 for identical twins. 



It is an extremely remarkable phenomenon. Why should the im- 

 munity mechanism be capable of distinguishing between distinct 

 (i.e. not identical) individuals? The way in which the individual mem- 

 bers of a species differ is in their genetic constitution. As we have 

 seen, the hereditary character of an individual, as derived from the 

 parents, is determined by the genes he inherits. The collection of 

 genes which any individual receives from each parent is determined 

 in the crossing-over process, which, as we have seen, gives him a selec- 

 tion of the genes carried by the parents. It follows that, except for 

 identical twins (which have arisen from one fertilized egg and there- 

 fore have the same genetic constitution), the chances are that prac- 

 tically every individual human being will be genetically different 

 from all others. This would not be the case in inbred animal colonies, 

 derived from a single pair, in which after a time the same or a nearly 

 similar genetic constitution may be established, but human commu- 

 nities have not reached this condition. 



These facts have been well understood for a long time. The sur- 

 prising and new finding is that the antibody mechanism can dis- 

 tinguish the differences of genetic constitution of individual mem- 

 bers of a species. An antibody reaction occurs whenever a foreign 

 gene is introduced. 



Since the antibody-forming mechanism is so sensitive, we should 

 now ask why it does not respond to the individual's own genes or 

 proteins. How does it know that a particular protein is its own; or 

 that a piece of tissue belongs to it and is not 'foreign'? This may arise 

 from the fact that the power of producing an immunological response 



