why blood group is defined as Positive or negative?

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The "Rh factor" on the surface of their red blood cells. Red blood cells contain D antigen.The D antigen is usually indicated by Rh positive (Rh+ does have the D antigen) or Rh negative (Rh− does not have the D antigen).Contain Rh factore of red blood cells which blood group is A+,B+,O+ etc. Without contain Rh factore of red blood cells blood which group is A-,B,-O- etc
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