Blood has two components: cells and plasma (liquid component). Blood group refers to the antigens (substances that stimulate antibodies) on red blood cells. These antigens make the blood in some group of individuals different from that of other groups.
Many of these blood groups have been discovered; however, the four common ones are: A, B, AB and O. they are inherited from parents. In the plasma circulating with the cells is antibody against other blood groups.
In addition to these blood groups, red blood cells also possess unique antigens known as Rhesus factors. The most important of these is Rhesus D (Rh D).
When the antigen is present on red blood cells, the red blood cells are said to be Rhesus positive but when the antigen is not present, they are known to be Rhesus negative.
Thus, blood groups are written combining the Rhesus factor with the ABO blood groups. So, an individual can have one of the eight possible blood groups:
Individuals with Rhesus positive red blood cells constitute the majority of the population, accounting for 80%-95% of population in most countries.
It is very important to have proper knowledge of your blood group in case of the need to undergo blood transfusion. This is to ascertain compatibility between the blood donor and the recipient. Receiving the wrong blood type can be life-threatening.
For example, someone with blood group A cannot receive blood from blood group B because the person with blood group A has anti-B antibody in the plasma. This antibody will react with the red blood cells and destroy them.
Blood group O does not have any antigen on the red blood cells, so no antibody can react with it. It can safely be transfused into anybody with other blood types, especially if the cells are separated from the plasma.
This is done for patients with other blood groups when their blood type is not available and there is urgent need to transfuse. Given the smaller volume of the plasma (with both anti-A and anti-B) being transfused with blood group O to the total plasma volume in the individual, this hardly results in serious complications.
However, caution needs to be taken while doing this. As they can potentially donate to anyone else, people with blood group O can only receive blood group O.
Conversely, as persons with blood group AB have both A and B antigens but no antibody in the plasma, they can receive blood from any of the groups in case of emergency; while they can only donate blood to their type only. This is because ant-A and anti-B antibodies in people with blood groups A, B and O will attack the antigens on the red blood cells and destroy them.
These scenarios have made blood group O to be tagged universal donor while blood group AB is tagged universal recipient. However, in the actual sense of it, the concept of universality only applies in case of emergency or near emergency situation. Otherwise, strictly compatible blood types should be transfused to those in need of transfusion to avoid blood transfusion reaction.
For Rhesus blood factors, blood with Rhesus negative can easily be given to recipients with both Rhesus positive and Rhesus negative. However, blood with Rhesus positive should not be given to recipients with Rhesus negative, except in dare emergency.
This is because, Rhesus positive red blood cells in an individual with Rhesus negative stimulates anti-Rhesus antibody production which can cause destruction of red blood cells with Rhesus positive, if transfused sometimes later.
Avoiding transfusion of Rhesus positive blood into an individual with Rhesus negative blood is even more important in women of reproductive age. If Rhesus positive blood is transfused into a woman with Rhesus negative blood, the Rhesus positive red blood cells stimulate anti-Rhesus antibody production.
When the woman becomes pregnant, these antibodies cross into the fetus through the placenta to attack Rhesus positive red blood cells in the fetus if the fetus is Rhesus positive. This results into a phenomenon known as Rhesus isoimmunisation.