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What is blood group systems – in forensic science
admin to Forensic Science  



The discovery of the ABO blood group system in 1900 marked the beginning of an era in which serology and blood, bloodstain, and body fluid evidence became invaluable in forensic science. With discovery of each new group, the potential for individualizing a bloodstain grew greater, although that goal was not reached using blood group systems. Blood group systems are based on antigens that are polymorphic, meaning that more than one variant exists, and at known frequencies in the population. For example, in the ABO system, the antigen is located on the surface of the red blood cell (RBC) and can be of Type A or B. Thus, the ABO system is polymorphic with known frequencies: 42 percent have Type A blood, 43 percent Type O, 12 percent Type B, and 3 percent Type AB. Like blood group systems discovered later, the ABO system is a genetic marker since the genes control which antigens are inherited and thus so is the blood type. For most of the blood group systems, the typing techniques used are similar to those used for ABO typing and include absorption-elution and absorption-inhibition.

Blood group systems are found in all components of the blood and in the body fluids of secretors as well. Antigen substances are found on the surface of the RBC, white blood cells (leukocytes primarily), and are associated with platelets. None of these antigens are found in such abundance as the AB antigens, so detection and typing of other blood group systems are more difficult and require larger samples. For whole, fresh blood this is not a great concern but in forensic cases where sample quality and quantity is limited, it presents severe limitations. The first blood group system identified after ABO was the MN system, discovered in 1927 by Karl Landsteiner and Levine. Soon it was realized that another marker Ss was closely related, and so the group is now known as the MNSs system. Frequency in the population is well dispersed, the most common type being MNSs at about 24 percent, ranging down to 1 percent for the NS type. While this seemed promising for forensic use, the system proved difficult to type and interpret.

The Rh system was identified in the 1940s (Landsteiner and Weiner) and has become known for its role in pregnancy and birth. Briefly, if a mother is type Rh-negative and gives birth to an Rh-positive child, she may develop antibodies to the Rh-negative factors. If she becomes pregnant with another Rh-positive child, the mother’s antibodies can harm that baby. The name Rh was derived from the Rhesus monkeys that were used in the early research. The system proved to be extremely complex with more than 40 antigens. Adding to the complexity was an inconsistent and non-standardized naming system. In the Fisher-Race system of naming, the six most common antigens are called C, D, E, c, and e. In forensic work, most tests focused on identifying the presence of D. Although the system has many variants and the antigens are fairly stable in bloodstains, the complexity of the system has limited forensic use.

A number of other so-called primary blood group systems have been identified, with the Kidd, Duffy, P, Kell, and Lewis systems having been used in forensic serology. However, none are easy to type in stains, and none are as persistent as the A and B antigens of the ABO system. More than 40 secondary blood group systems have also been discovered, but none have been used forensically. Research into typing techniques for forensic work faltered once the isoenzymes were discovered and simple typing techniques using electro-phoresis were developed for the isoenzyme genetic markers. However, even isoenzyme systems have given way to DNA typing, which is much more successful in individualizing blood than isoenzymes or blood group systems ever were.