Blood Groups

Blood Groups

It is recommended that before reading this section you read the 'Blood Components' Section.
What makes a person's blood group A or blood group B? What is the meaning of positive and negative? What makes two different bloods from two different persons compatible (match) or incompatible (do not match)?
The system of A’s and B’s is called the ABO or the Landsteiner blood grouping system.

The body, which is continuously being bombarded by bacteria, viruses and foreign substances, has a complicated defense mechanism. One way of protecting itself is through internal substances formed in the blood, to destroy any foreign substances that enter the blood stream. The substances formed by the body in the blood are called antibodies (agglutinins) and the foreign substances are called antigens. For example, pollen from flowers, micro-organisms such as bacteria and viruses, proteins molecules from animals, foreign tissues like transplanted organs and blood transfusions, and penicillin are examples of foreign antigens. However antigens can be also internal (intrinsic). These internal antigens are the personal characteristics of a person or a group of persons, and we are born with these internal antigens.

On the surface of a red blood cell (the erythrocyte) there are many layers of substances among which are the (internal) antigens. There are two important antigens, which are called the A antigen, and B antigen. If on the surface of the red blood cells there is the A antigen, the blood group is A; if there is the B antigen, the blood group is B; if there are both A and B antigens, the blood group is AB. If there is neither the A nor the B antigen on the red blood cell, the blood group is O. It is these antigens that are responsible whether a blood is A, B, AB or O.

Blood with the A antigen on the red cells has B antibodies in the plasma. Blood with the B antigen on the red blood cell has A antibodies in the plasma. Blood with A and B antigen on the red cell has no A or B antibodies in the plasma. Blood with no A and B antigens on the red blood cell has A and B antibodies in the plasma. This means that a blood type has the opposite antibody in the plasma.

This concept is not very easy to understand and quite difficult to explain. Keep the following in mind, if A antigens meets A antibodies, A antibodies destroy A antigens. On the same principle, if B antigens meet B antibodies , B antibodies destroy B antigens . This would cause the red blood cells that are being attacked to clog together and of course block the veins and arteries of the patient.

Follow carefully these examples:

Legend: Patient or recipient is red and donor is blue.

Rule of thumb: When there is a reaction (i.e. bloods that do not match) the patient plasma antibodies attack the antigen on the red blood cell of the donor. The antigen on the donor's red blood cells are foreign antigens to the patient (recipient).

If a patient of blood group A (A antigen, antibody B) receives blood group B (B antigen, antibody A) what happens? The antibody B of the patient would attack the B antigen of the donor causing them to stick together. This would block the veins of the patient causing a very serious condition to the patient which might result in death. The same would happen if patient blood B receives blood A. Try to work it out. Therefore blood group A can be given only to persons whose blood is either A and AB only. On the same lines, blood group B can be given to persons whose blood group is B and AB.

If a patient of blood group O (no antigens, A & B antibodies) receives blood group B (B antigen, antibody A), B antibodies of the patient would attack the B antigen of the donor causing the red blood cells to stick or clog together. On the same principles, if blood group A were transfused to a blood group O patient, the A antibodies of the patient would attack A antigens of the donor. Therefore blood group O patient can receive blood from blood group O donor only.

If a patient of blood AB (A and B antigen, no antibodies) receives any group of blood nothing happens because there are no A or B antibodies in the plasma of the patient (recipient) to attack the A or/and B antigen of the foreign blood.

The following figure summaries the above:O universal donor, AB universal recipient.

Donor
O	OK	OK	OK	OK
A	NO	OK	NO	OK
B	NO	NO	OK	OK
AB	NO	NO	NO	OK
	O	A	B	AB	Recipient

For those who find the above table confusing, keep this in mind:

O can give to all
A can give to A and AB
B can give to B and AB
AB can give to AB only

O can receive from O only
B can receive from B and O
A can receive for A and O
AB can receive from all

The Rhesus Factor

There is another group of factors that effects compatibility, the Rhesus (Rh) factors where the Rh D antigen is the most significant. This is another antigen on the surface of the red blood cell. The name Rhesus is derived from the type monkey that was involved in the experiments to study these factors. When this Rhesus antigen is present the blood group is described as positive. When the Rhesus factor is absent the blood group is described as negative. Therefore there is blood group O Rh Positive and O Rh Negative, A Rh Positive and A Rh Negative, B Rh Positive and B Rh Negative, and AB Rh Positive and AB Rh Negative. As a general rule the Rh negative group can give safely the correspondent Rh positive group. However the Rh positive group is not recommended as a donor to the Rh negative group patient (recipient). This is not always the case because Rhesus positive blood may be given to Rhesus negative blood together with special drugs and closer medical supervision only in emergencies. However transfusing Rhesus positive blood into a Rhesus negative patient is avoided, especially in young females who are potential mothers. Transfusing Rhesus positive blood to Rhesus negative females will instigate the formation of antibodies against the Rh D antigen in the mother's blood. This would cause problems during pregnancy and to the newborn baby causing heamolytic disease of the newborn.