THE TRANSPORTATION SYSTEM 



511 



blood some difficulties have been encoun- 

 tered. 



Blood types M and N. By injecting the red 

 blood corpuscles of a person into a rabbit, 

 a specific antibody is built up in the rabbit 

 which when mixed with more red cells of 

 the same individual will cause them to ag- 

 glutinate or stick together in clumps, 

 a reaction that is easily visible under the 

 microscope, or even by the naked eye for 

 that matter. This is a clear-cut test, not 

 greatly unlike the precipitin test, which 

 can be used to determine whether or not all 

 human blood is identical. This has been 

 done in thousands of people and it is now 

 known that blood is not all alike. In fact, 

 the evidence today indicates that there are 

 a great many different kinds of blood types. 

 We shall consider only a few, at this mo- 

 ment, the M and N forms. 



By testing thousands of human beings it 

 has been discovered that there are two dif- 

 ferent kinds of proteins in the red blood 

 cells tliat will cause antibody formation in 

 rabbits, the M protein and the N protein. 

 Every human being possesses either one or 

 the other or both. Fortunately, these pro- 

 teins do not produce their corresponding 

 antibodies in the blood, which makes it un- 

 necessary to determine which of these types 

 a person has before a transfusion is permit- 

 ted. However, this knowledge has had some 

 value in determining questionable cases of 

 parenthood. It so happens that the type 

 of protein, M or N or both, is inherited in a 

 definite Mendelian fashion. 



The Rhesus factor. Recently another 

 group of red cell proteins have been dis- 

 covered, designated as the Rhesus or simply 

 the Rh factor because it was first discov- 

 ered in the Rhesus monkey. A survey of 

 various populations showed that it occurs 

 in about 85 per cent of the people. If a per- 

 son possesses the protein he is said to be 

 Rh positive, whereas if his red cells do not 

 contain the factor he is Rh negative. Nor- 

 mally there is no anti-Rh in the serum, and 

 of course no difficulty is encountered unless 



onfibody 

 Tormed 



Fig. 19-15. If Rh positive blood is transfused into an Rh 

 negative person, anti-Rh is built up in the latter's 

 blood, if, at some later time, another transfusion of 

 Rh blood is given, complications may arise because 

 the anti-Rh will bring about the clumping of red cells. 



