There isn't a single blood group that's inherently "very weak." Instead, the weakness refers to the expression of certain blood group antigens on the surface of red blood cells. This means the antigens, which are molecules that trigger an immune response, might be present in very low numbers or have altered structures. This can result in weaker reactions during blood typing tests.
Several blood groups can exhibit this weak expression. Examples include weak variants of blood group A and B, such as:
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Weak A subgroups: These include A3, Aend, Ax, Am, Ay, and Ael. These variants have reduced antigen expression compared to the standard A blood group. (Detection of a weaker subgroup of A in ABO blood group system)
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Weak B subgroups: These are rarer than weak A subgroups. One example is the Bm phenotype, which shows very weak B activity. (A case of weak blood group B expression (Bm) associated with ..., A Case of Weak Blood Group B Expression (Bm) Associated with ...)
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Weak D (Rh): This refers to a weakened expression of the D antigen in the Rh blood group system. There are several types of weak D, some of which can be challenging to detect with standard tests. (Weak D type 1.1 exemplifies another complexity in weak D genotyping, 6007 - Gene ResultRHD Rh blood group D antigen [ (human)], Molecular genetics of the Rh blood group system: alleles and ...)
The inheritance of these weak phenotypes usually follows Mendelian genetics, but there can be exceptions. (THE UNUSUAL WEAK blood group A and B activities on red blood cells have been found in various populations.) The presence of these weak expressions is clinically significant as it can lead to discrepancies in blood typing and potential transfusion complications if not properly identified.
