The most significant Rh antigen is the D antigen, because it is the most likely to provoke an immune system response of the five main Rh antigens. It is common for D-negative individuals not to have any anti-D IgG or IgM antibodies, because anti-D antibodies are not usually produced by sensitization against environmental substances. However, D-negative individuals can produce IgG anti-D antibodies following a sensitizing event: possibly a fetomaternal transfusion of blood from a fetus in pregnancy or occasionally a blood transfusion with D positive RBCs. Rh disease can develop in these cases. Rh negative blood types are much less common in Asian populations (0.3%) than they are in European populations (15%).The presence or absence of the Rh(D) antigen is signified by the + or − sign, so that, for example, the A− group is ABO type A and does not have the Rh (D) antigen. ABO and Rh distribution by country
As with many other genetic traits, the distribution of ABO and Rh blood groups varies significantly between populations. Other blood group systems
As of 2021, 41 blood-group systems have been identified by the International Society for Blood Transfusion in addition to the ABO and Rh systems. Thus, in addition to the ABO antigens and Rh antigens, many other antigens are expressed on the RBC surface membrane.
2
          
          
            FeSO
            
              4
            
            
              
            
          
          
            
              →
              
                Δ
              
            
          
          
            Fe
            
              2
            
            
              
            
          
          
            O
            
              3
            
            
              
            
          
          +
          
            SO
            
              2
            
            
              
            
          
          +
          
            SO
            
              3
            
            
              
            
          
        
      
    
    {\displaystyle {\ce {2FeSO_4->[\Delta]Fe_2O_3 + SO_2 + SO_3}}}
  Like other iron(II) salts, iron(II) sulfate is a reducing agent.