Blood groups and blood testing


Blood groups and blood testing

The blood groups

Blood group antigens exist on the surface of the red cell membrane (see also p. 4). There are numerous blood group systems encoded by genes on different chromosomes. They are highly variable in their polymorphism and clinical significance.

The most important blood group is the ABO system. The genes encoding the ABO antigens are located on chromosome 9 and are inherited in an autosomal dominant fashion. Each antigen is a sugar residue made by a specific glycosyl transferase. The ABO system is crucial in clinical blood transfusion as there are naturally occurring IgM antibodies in the serum targeted against the non-present ABO antigens (Table 41.1). These antibodies necessitate the use of ABO ‘compatible’ blood for transfusion. For example, the administration of incompatible group A blood to a group B patient would engender a potentially fatal haemolytic transfusion reaction due to the destruction of the donor’s group A red cells by the recipient’s anti-A antibody.

In other blood group systems ‘naturally occurring’ antibodies are rare. However, ‘immune antibodies’, usually of IgG type, may be induced by transfusion of blood expressing different blood group antigens or maternal exposure to fetal red cell antigens. Where such immune antibodies are present, transfused blood must be matched for the relevant blood group system in addition to ABO. Maternal formation of immune antibodies against antigens of the Rhesus (Rh) blood group system, particularly the strongest antigen D, accounts for most cases of haemolytic disease of the newborn (p. 90).

The testing of blood

Donor blood

The safety of blood transfusion is maximised by careful selection of donors. All donors should be in good health and, wherever possible, unpaid volunteers. Particular care is taken to exclude potential donors who may harbour infective diseases which are transmissible by blood transfusion – thus people with recent jaundice (? hepatitis), a history of recent travel to malarial areas or risk factors for HIV or Creutzfeldt–Jakob disease (CJD) infection are not suitable donors.

The objective of routine testing of donated blood is to provide blood which can be selected for likely compatibility with a patient and which contains no identifiable infectious agent (Table 41.2). Antibody testing (e.g. for HIV and hepatitis C) is now supplemented by molecular techniques sensitive enough to trace the virus in the blood before the development of antibodies (i.e. during the ‘window period’).

Jun 12, 2016 | Posted by in HEMATOLOGY | Comments Off on Blood groups and blood testing
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