BLOOD GROUP ANTIGENS 



93 



lAiii.i-; 7-,i 



Possible (lenetic I'athways for the Production of Blood Group Substances. II" 



Precursor substan 



+ Inconvertcd I.( 



Rir Asubsl. Bsubsl. 

 +11 +11 



Le"" nonsecrctor 



ways they act to modify the precursor substance, a mucopolysac- 

 charide which is believed to be identical with the material found in 

 the secretions of the individuals who secrete neither A, B, or H nor 

 Le^ or Le** substances. The U gene acts to add a-fucosyl units to 

 this precursor substance, and the S' gene adds still more. The B 

 gene adds a-galactosyl units, and the A gene adds a-galactosaminoyl 

 units (see Table 7-2). 



The scheme of Table 7-2 is inadequate in some respects, and 

 Watkins and Morgan suggest replacing it by the more complicated 

 system shown in Table 7-3. 



Other Human Red Cell Receptors 



Bauhinia Receptor 

 In addition to the red cell receptors characteristic of the various 

 blood groups, there are a number of receptors, some common to all 



