Blood groups

The circulatory system consists of the heart, blood vessels and blood. The bloodstream carries oxygen and nutrients to each cell, and picks up waste products (such as carbon dioxide) for elimination. The bulk of blood is the liquid plasma. Floating in this plasma are red blood cells that carry oxygen, white blood cells that form part of the immune system, and clotting agents called platelets. There are different ways to classify blood. The two major forms of classification include the ABO system and the Rhesus (Rh) type system, characteristics that are inherited independently. Together, they comprise the eight main blood groups. Other blood group systems exist and, to date, researchers have identified around 300 minor factors.

The ABO group
The four different blood groups are A, B, AB and O. A person's blood group is determined by a pair of genes, one each inherited from their mother and father. Each blood group is identified by its own set of complicated chemical substances - called antigens - located on the surfaces of red blood cells. When a person needs a blood transfusion, it is important that the donated blood matches their particular blood group. A mismatch can cause serious complications

The Rhesus factor
A person's Rhesus type is also determined by a pair of genes, each one inherited from one parent. Blood is either Rh-positive or Rh-negative, depending on whether or not certain molecules are present. A person who is Rh-negative will experience a severe immune system reaction if Rh-positive blood gets into their bloodstream. This can happen during childbirth, if an Rh-negative woman gives birth to an Rh-positive baby. If blood cells from the baby travel across the placenta, the woman's immune system will regard the Rh-positive cells as a threat. Specialised white blood cells will make antibodies designed to kill Rh-positive blood cells. If the woman subsequently conceives another Rh-positive baby, her immune system will flood her child with antibodies. These antibodies then destroy the baby's red blood cells. If left untreated, this can result in severe anaemia or even death.

Preventing Rhesus disease
Rhesus disease is now rare, since Rh-negative mothers who give birth to Rh-positive babies are immunised within 72 hours of giving birth. The immuneglobulin preparation works by killing the baby's red blood cells inside the mother's bloodstream before her immune system has time to react.

Blood transfusion
A blood transfusion is the transfer of blood from one person to another. The donated blood must match the recipient's blood type, or complications will occur. Generally, both receiving and donating blood are safe medical procedures. For instance, O negative blood can be given to anybody if necessary, but it is always preferable to match the exact blood group. The different types of blood transfusion include homologous (whole blood transfusion) and aphaeresis (only certain components - such as platelets - are transfused).

Blood groups in Australia
A person's blood group is described by the appropriate letter (A, B, AB or O) and by whether or not their blood is Rhesus positive or negative. According to the Red Cross, the percentage of blood group frequency in Australia is:

• O positive - 40 per cent
• O negative - 9 per cent
• A positive - 31 per cent
• A negative - 7 per cent
• B positive - 8 per cent
• B negative - 2 per cent
• AB positive - 2 per cent
• AB negative - 1 per cent

• Your doctor
• Australian Red Cross Blood Service Tel. 131 495

• The two major classifications of blood include the ABO system and the Rhesus type system.
• The four blood groups are A, B, AB and O, and each can be Rhesus positive or Rhesus negative.
• Donor blood must match the recipient's blood group, or serious reactions can occur.

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