Kidd Blood Grouping System

Bloodstains left behind at the scene of a crime or found on a suspect's clothing can be a valuable source of identification . Red blood cells contain a number of proteins known as antigens on their surface. The exact biochemical nature of these antigens is inherited and so varies from person to person. An individual's blood can be typed according to several different classes of antigen . One of these is the Kidd system, which was discovered in 1951.

There are three major Kidd blood groups, depending on which combination of variants of a protein called Jk a person has on their red blood cells. The two variants of Jk are called Jk ^a and Jk^b. The combinations are Jk^a Jk^a, Jk^a Jk^b, and Jk^b Jk^b. Blood from people in the first group may contain antibodies against Jk^b and that from those in the third group may contain antibodies against Jk^a. An antibody is a protein component of the immune system , which binds to a relevant antigen and plays a part in destroying foreign cells or bacteria. In blood transfusion, the first and third Kidd types would be incompatible, because antibodies from one blood type would bind to the matching antigen on the other blood type and the cells would clump together.

Antisera, that is, reference blood samples containing a specific type of antibody, are used to find out the type of blood present in a blood stain. They react with antigens and cause a clumping reaction that is measured by various techniques. In the case of Kidd blood typing, an antiserum containing Jk^a antibodies would bind to Kidd types containing Jk^b antibodies. The first blood type system identified was the ABO system, where people have one of four types of blood: A, B, AB, or O, depending on which combination of genes affecting these antigens they inherit.

Besides Kidd and ABO, other blood typing systems include Rhesus, MNS, Kelly, Duffy, and Lewis. They are all inherited independently. Therefore someone with blood group O could have any of the three different Kidd types. The more blood types that can be co-analyzed on one sample, the more individualizing it becomes as evidence . Coincidental matches of blood found at the scene of a crime with that associated with a suspect become less likely with the number of typing reactions carried out. However, the amount of blood present in a stain may limit the number of typing determinations that can be done. Forensic blood typing is perhaps most useful for eliminating a suspect than, on its own at least, identifying one.

see also Serology.