Blood Groups (Human Biology, Human Anatomy & Physiology)
Reminder about antigens and antibodies:
Antibodies are proteins produced by the body's immune system. They attack antigens on certain cells such as those on harmful bacteria that invade the body.
Antibodies are specific to particular types of cells (more accurately, to particular antigens).
For example, an antibody that attacks the antigens on a particular bacterium would not attack bacteria in general i.e. it would attack the specific bacteria, not any bacteria.
Blood groups are defined according to the antigens and antibodies present in blood.
There are many types of blood and several blood group systems used to classify blood types (the terms 'blood types' and blood groups' are widely used interchangeably).
The best known and most widely used blood group systems include the ABO blood group system and the Rh blood group system.
- The ABO blood group system is easy to explain because it is based on just two antigens (A and B) being either present, i.e. attached to the red blood cells (RBCs), or absent.
- The Rh (Rhesus) blood group system is more complicated because it involves consideration of the presence or absence of 50 defined blood-group antigens, of which the five antigens D, P, c, E, and e are the most important.
- A simple combination of the ABO blood group system with the Rh blood group system is widely used. This provides more information than the ABO system alone, while retaining enough simplicity for ease of use and understanding by many people - see below.
ABO blood group system
According to the ABO blood group system, blood can be classified into one of the four major blood types, which are:
A, B, AB and O.
The symbols A, B, AB and O used to represent the four major blood types indicate which antigens (just A, just B, both A and B or neither A nor B) are present on the red blood cells.
Blood that contains red blood cells of a particular type e.g. group A blood contains red blood cells with antigen A attached, also has (or does not have) particular antibodies present in the blood plasma.
ABO blood groups indicate the combinations of antigens and antibodies present in the blood as follows:
Antigen(s) on RBCs
Antibody / antibodies in plasma
A and B
neither A nor B
both anti-A and anti-B
Table 1: Antigens in RBCs and Antibodies in plasma for each of the 4 main blood groups
Antibodies are specific. That is why the antibodies to antigen B (labelled 'anti-B' in the table above) in the blood plasma of group A blood do not attack the A antigens on the red blood cells of group A blood. However, the anti-A antibodies in group B blood plasma would attack the A antigens on the red blood cells of of group A blood if group B blood were given to a person whose blood type was group A.
Using the information in the table summarizing the antigens and antibodies in each of the four main blood types (above) it is possible to work out which types of blood can be safely donated to or received from people who have each of the four main types of blood.
Can receive blood from:
Can donate blood to:
A and O
A and AB
B and O
B and AB
any of these groups
any of these groups
Table 2: Blood Transfusion between each of the 4 main blood groups
Note that the above only takes into consideration the two antigens A and B and (antibodies to them), according to the ABO blood group system. In reality, various safety checks would normally take place prior to the donation or receipt of blood.
For example, other possible antigens and antibodies may be taken into consideration and donated blood screened for transfusion transmitted infections (TTIs), the most common diseases transmitted via blood being viral infections such as HIV (see HIV in the news) and forms of hepatitis (see hepatitis in the news).
Rh (Rhesus) blood group system (simple version)
Although there are many Rh system antigens, the simplest and most common use of the Rh blood group system is in combination with the ABO blood group system. In this case each of the four ABO blood groups A, B, AB and O are divided into two groups because each can be either Rhesus 'positive' (indicated by '+'), or Rhesus 'negative' (indicated by '-'). The Rhesus positive groups A+, B+, AB+ and O+ have the D antigen of the Rh blood group system, which is also known as 'the Rhesus factor', attached to red blood cells (RBCs). Conversely, the Rhesus negative groups do not have the Rhesus factor attached to RBCs.
Therefore, according to the (simple version of) the Rh blood group system, there are the following eight (8) blood groups:
- A+ , also called "A, Rhesus positive"
- A- , also called "A, Rhesus negative"
- B+ , also called "B, Rhesus positive"
- B- , also called "B, Rhesus negative"
- AB+ , also called "AB, Rhesus positive"
- AB- , also called "AB, Rhesus negative"
- O+ , also called "O, Rhesus positive"
- O- , also called "O, Rhesus negative"
In all cases the "positive" means that, in addition to any other antigens present in the case of the ABO group concerned, the Rhesus Factor, which is the most immunogenic D antigen of the Rh blood group system is also attached to the red blood cells.
The Rhesus Factor is sometimes referred to as Rhesus D, RhD antigen or (when the context of the Rh blood group system is obvious), simply the D antigen.
Examples of importance of blood groups
Blood groups matter when there is a possibility of a person (or other animal for which blood groups are important, e.g. equines, pigs, rodents) receiving antibodies to antigens attached to their red blood cells.
The main situations in which this might apply - hence blood tests are commonly used to determine blood type (blood group) are:
- Re. Blood Transfusions:
- Blood banks store blood according to blood type (blood group)
- Patients should ideally be carefully checked for blood type, receive only their own blood or a type-specific match, and if possible have donated blood cross-matched before receipt to minimize the risk of a transfusion reaction. This is not always possible, especially e.g. in emergency situations in remote areas.
Note: Jehovah's Witnesses (JWs) do not accept blood transfusions for religious reasons. Their decision in this matter is increasingly respected and has lead to medical advancements, initially to accomodate JWs, but also of benefit to others:
"many of the techniques developed for use in Jehovah's Witness patients will become standard practice in years to come"
from 'Continuing Education in Anaesthesia', Critical Care & Pain, 2004, Volume 4, No. 2, page 39 - reference from http://bit.ly/10rRFBE.
- Re. Pregnancy:
There may be concern due to, and action(s) taken against, the risk of complications due to mis-match between a pregnant woman's blood type and that of a fetus she is carrying.
Note: The ABO blood group system is easy to explain and is included in many school biology courses e.g. GCSE Biology (UK). The Rh blood group system is also important and widely used but may not be included in some first-level courses in biology and/or human anatomy and physiology.