Biology of Man 525 



type AB are known as "universal recipients" because they can receive 

 blood from any type. 



Additional agglutinogens, known as M and N, have been found in 

 human erythrocytes, but no corresponding agglutinins are reported. 

 These M and N agglutinogens are inherited independently of the pre- 

 vious groups and are useful in identifying bloods but need not be con- 

 sidered in blood transfusions. 



A third type of agglutinogen is known as the Rh factor because it 

 was first discovered in the blood of the Rhesus monkey. The erythro- 

 cytes in about 85 per cent of the white people contain Rh agglutinogen; 

 hence such persons are Rh positive; the remaining 15 per cent are Rh 

 negative. Under normal conditions no agglutinin (antibody) is present 

 in the blood plasma to react with the Rh agglutinogen. However, if 

 an Rh-negative (Rh— ) person receives Rh-positive (Rh+) blood by 

 transfusion, there will be formed in the plasma of the recipient some 

 Rh-positive agglutinins (anti-Rh-positive antibodies). When this re- 

 cepient receives a second quantity of Rh-positive blood, the previously 

 formed Rh-positive agglutinins will react with the Rh-positive agglutino- 

 gens (of second transfusion) with serious reactions. The Rh-positive 

 agglutinogen was at first thought to be inherited as a dominant factor, 

 but recent studies suggest that instead of just two types (Rh-positive and 

 Rh-negative) there are eight or more alleles which may result in many 

 genetically different combinations. 



In human pregnancies, a Rh-negative mother and a Rh-positive fa- 

 ther may have a Rh-positive offspring (inherited from its father). The 

 Rh-positive factor of the embryo while in the mother may pass by blood 

 through the placenta to stimulate the mother to produce Rh-positive 

 agglutinins (antibodies). If the same two parents conceive a second 

 child, the mother may pass some of her Rh-positive factor through the 

 placenta to this second Rh-positive child and the reactions may cause 

 destruction of erythrocytes in the latter. If extreme, the embryo may 

 die prenatally (anemia) or it may die postnatally, but if not serious the 

 infant may recover. 



The inheritance of the type of blood (O, A, B, or AB) is due to a series 

 of alleles (genes) : allele A produces agglutinogen A; allele A^ produces 

 agglutinogen B; allele a produces no agglutinogen, and the latter is re- 

 cessive to the other two. Neither gene A nor A^ is dominant to the other. 

 When both A and A^ are present, an individual of the AB group results. 

 Blood types are inherited specifically and do not change; hence, blood 

 tests may be used in certain cases of disputed parentage. These blood 



