90 Inside the Living Cell 



other hand the serum of A and B is not capable of agglutinating cor- 

 puscles of type O, because group O corpuscles are not antigens for 

 the A and B antibodies. 



The blood of group AB contains no antibodies, so that an indi- 

 vidual with this blood can receive blood from any other group with- 

 out harmful effects. On the other hand, if it is given to an individual 

 having blood of any other group, it will be agglutinated because all 

 these bloods contain its antibodies. 



This is a very complicated situation and we may wonder how it 

 came about. The different blood groups are controlled by genes, so 

 that the blood group of each person is determined by that of his 

 parents in accordance with the Mendelian laws of heredity. It would 

 seem likely that the present blood groups are derived from quite dis- 

 tinct ancestral strains of the human race, but they go back to a stage 

 long before the development of the present races of mankind as the 

 distinction of blood groups is not to any great extent a matter of race. 



It would be expected that if an individual inherited from his 

 parents two incompatible components of blood, he would not sur- 

 vive and that, in the course of time, genie incompatibility would be 

 eliminated. However, some other antigenic components of blood 

 have been recognized, which are also inherited by Mendelian rules. 

 One of these is the so-called 'Rhesus factor' which has been found 

 to be present in 83 per cent of the population of England and 

 about 85 per cent of the white population of New York, and may 

 lead to incompatibility between the blood of the unborn child and 

 that of the mother if she does not possess it and her husband does. 

 The human placenta is not a perfect barrier against the passage of 

 antibodies and in some cases antigens from the foetus may pass into 

 the blood stream of the mother and there give rise to the formation of 

 antibodies. If these find their way back into the child, they produce 

 a disastrous reaction (haemolytic disease of the new born). 



Another example of the immunity mechanism being too efl&cient 

 has been found in attempts to graft skin from one person to another. 

 An individual can replace a certain amount of lost skin by growth 

 from the edges of the wound inwards. If the area of skin lost is very 

 great, this process is rarely successfully completed. The task of 

 making so much new skin is too much for the resources of the injured 

 person. It is possible to help the growth of new skin by grafting skin 

 from other uninjured parts of the individual's own body. This can 

 be taken off in thin layers which do not cause a serious injury and 

 they help to cover the wound and act as centres from which new 

 skin cells are formed. It would be a great help if skin taken from 

 other individuals could also be grafted. It would then be possible to 



