Rhesus and other Antigens

After the ABO system, the most important blood group system is the Rhesus system. There are several different antigens in the rhesus system. There are 5 important ones which are tested for: D, C, c, E, e. The strongest of these is Rh (D) which is the antigen tested in blood grouping. If the Rh (D) antigen is present, it is labelled Rh+ve.1 For example, if a person has the A antigen and Rh antigen, they are A+. If they have the A antigen but no rhesus antigen, they are A-. 

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If a person with rhesus negative blood is exposed to blood which is rhesus positive (either from transfusion or through pregnancy), the person's immune system will recognize the Rh antigen as foreign, and develop IgG antibodies against the Rh antigen. Therefore, if the same patient was to receive Rh+ve blood again, their blood would contain antibodies against the donor blood cells, and a haemolytic transfusion reaction may occur. This can occur in the context of pregnancy - you can read about Haemolytic Disease of the Newborn on the Haemolytic Anaemia page. You can read about haemolytic transfusion reactions on the Transfusion reactions page. 

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There are other antigens that can be present on the red cell surface. However, naturally occurring serum antibodies against antigens which are not present is less common than the ABO antigens.1

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Inheritance of Blood Groups

Blood groups are inherited from parents. The A and B alleles are both dominant alleles, and the O allele is recessive. This means that if a person inherited an A allele from one parent, and an O allele from the other parent, their blood group would be A. If they inherit an O allele from both parents, they will be O.1 

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Blood Components

Several blood products exist and can be ordered for various indications. To understand what blood product to order for which indications, it is important to understand the components of blood. Whole blood contains:

1) Red blood cells

2) Buffy coat - white blood cells and platelets

3) Plasma - water, proteins (clotting factors, antibodies), electrolytes, fibrinogen 

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Transfusion of Blood Components

Red Cells

Red blood cells are usually washed so that they contain very little plasma and white blood cells. This is to prevent allergic reactions to the proteins in plasma, and to prevent fevers caused by donor white blood cells. Each unit of red blood cells raises the Hb by approximately 1g/dL. Red cell transfusion must be ABO Rh compatible to prevent haemolytic transfusion reactions, which can be fatal.2

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Platelets

Platelet transfusions are not as common, because their half-life in the blood is very short (2-3 days). If a patient has a low platelet count but is going to have an invasive procedure, platelet transfusions can be given to prevent excessive bleeding. The important thing to note about platelet transfusions is that they are CONTRAINDICATED in patients with Thrombotic Thrombocytopenic Purpura (TTP), and Heparin-Induced thrombocytopenia (HIT). 

• TTP occurs due to deficiency of ADAMTS13, which is a VWF cleaving protein. Platelets binds to vWF and form platelet-rich thrombi, which means these patients are at risk of stroke. Giving more platelets will INCREASE this risk.

• HIT is an immune-mediated response to heparin, where IgG proteins stick on to the surface of the platelets. The antibodies then cause the aggregation of platelets, forming thrombi, and increasing the risk of thromboembolism. Therefore giving more platelets would INCREASE this risk.3,4

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Fresh Frozen Plasma (FFP)

Plasma contains clotting factors, so it can be used to replace lost clotting factors in DIC, or to reduced bleeding in major haemorrhages. When the Major Haemorrhage protocol is activated for a patient with severe bleeding (> 40% of total blood volume), 4 units of FFP are given.2

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Cryoprecipitate

This is rich in fibrinogen (which can be converted to fibrin). Fibrin is the end of the coagulation cascade - it forms a mesh for platelets to embed, thereby reducing bleeding.  2 units of cryoprecipitate are used as part of the major haemorrhage protocol.2

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Prothrombin Complex Concentrate

This contains a mixture of vitamin-K dependent clotting factors, and therefore can be used for emergency reversal of warfarin.2,5