The composition of blood varies between individuals. This means that each blood group has it own precise composition and the difference in structure is what makes a person’s blood type. Most people use the ABO system to group blood types, as well as rhesus factor, which is either positive or negative. Do you know your blood group and the facts about compatibilities. This article will explore more information about blood groups and dive more into the facts about compatibilities.

What makes a blood type?

The main components of blood are:

• red blood cells, which carry oxygen around the body
• white blood cells, which play a crucial role in the immune system
• plasma, which is a yellowish liquid that contains proteins and salts
• platelets, which enable clotting

The blood group will depend on which antigens are on the surface of the red blood cells.
Antigens are molecules. They can  be either proteins or sugars. The types and features of antigens can vary between individuals, due to small genetic differences.

The antigens in blood have various functions, including:

• transporting other molecules into and out of the cell
• maintaining the structure of red blood cells
• detecting unwanted cells that could cause illness

Scientists use two types of antigens to classify blood types:

• ABO antigens
• Rh antigens

Antigens and antibodies play a role in the immune system’s defense mechanism.

ABO and the most common blood types

The ABO blood group system classifies blood types according to the different types of antigens in the red blood cells and antibodies in the plasma.
They use the ABO system alongside the RhD antigen status to determine which blood type or types will match for a safe red blood cell transfusion.

There are four ABO groups:

Group A: The surface of the red blood cells contains A antigen, and the plasma has anti-B antibody. Anti-B antibody would attack blood cells that contain B antigen.

Group B: The surface of the red blood cells contains B antigen, and the plasma has anti-A antibody. Anti-A antibody would attack blood cells that contain A antigen.

Group AB: The red blood cells have both A and B antigens, but the plasma does not contain anti-A or anti-B antibodies. Individuals with type AB can receive any ABO blood type.

Group O: The plasma contains both anti-A and anti-B antibodies, but the surface of the red blood cells does not contain any A or B antigens. Since these antigens are not present, a person with any ABO blood type can receive this type of blood.

Blood Groups and Compatibilities facts

There are many blood groups in the human population with its compatibilities facts. The most important of these are ABO and RhD.  Transfusion with ABO incompatible blood can lead to severe and potentially fatal transfusion reactions.  RhD is highly immunogenic and can lead to red cell haemolysis in certain settings. 

ABO antigens and antibodies.

The ABO blood group is the most important of all the blood group systems.  There are four different ABO blood groups (see Table1), determined by whether or not an individual’s red cells carry the A antigen, the B antigen, both A and B antigens or neither.
Normal healthy individuals, from early in childhood, make red cell antibodies against A or B antigens that are not expressed on their own cells.  These naturally occurring antibodies are mainly IgM immunoglobulins.  They attack and rapidly destroy red cells carrying the corresponding antigen.  For example, anti-A attacks red cells of Group A or AB. Anti-B attacks red cells of Group B or AB.

If ABO incompatible red cells are transfused, red cell haemolysis can occur.  For example if group A red cells are infused into a recipient who is group O, the recipient’s anti-A antibodies bind to the transfused cells.  An ABO incompatible transfusion reaction may result in overwhelming haemostatic and complement activation, resulting in shock, renal failure & death

Rhesus D  (RhD) antigen

There are more than 40 different kinds of Rh antigens.  The most significant Rh antigen is RhD.  When RhD is present on the red cell surface, the red cells are called RhD positive.  Approximately 80% of the Australian population are RhD positive.  The remaining 20% of the population that lack the RhD antigen are called RhD negative.

Antibodies to RhD develop only after an individual is exposed to RhD antigens via transfusion, pregnancy or organ transplantation.  Anti RhD (or anti-D) antibodies destroy RhD positive red cells and can lead to haemolytic transfusion reactions.  This is of particular importance in pregnancy where anti-D antibodies can cross the placenta from mother to unborn child and lead to haemolytic disease of the newborn.
As a general rule, RhD negative individuals should not be transfused with RhD positive red cells, especially RhD negative girls and women of childbearing age. If transfusion of an RhD positive product to RhD negative recipient is unavoidable a haematologist should be consulted and administration of anti-D immunoglobulin considered.

Are they compatible?

When a transfusion is given, it is preferable for patients to receive blood and plasma of the same ABO and RhD group.  However if the required blood type is unavailable, a patient may be given a product of an alternative but compatible group as shown below. 

Note that Group O RhD negative (O negative) red cells have neither ABO nor RhD antigens on their surface.  O RhD negative red cells are issued in emergency situations where life saving transfusion is required prior to completion of a crossmatch.  Both RCH and RWH blood banks maintain a reserve of 5 emergency O RhD Negative red cells.  Group O is often referred to as the universal red cell donor.
Group AB individuals have neither anti-A nor anti-B antibodies in their plasma. Group AB plasma can therefore be given to patients of any ABO blood group and is often referred to as the universal plasma donor.

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