Blood is an essential part of the body; it transports oxygen and nutrients to cells, removes waste products, transports immune cells and helps us to regulate our body temperature. Diseases of the blood, or loss of large amounts of blood, can lead to life-threatening situations and in these situations blood transfusions may be required. (See the NHS blood donor website for more information on when transfusions are needed, http://www.blood.co.uk/about-blood/components/#redbloodcells). Whilst the transfusion of blood has become a relatively straight-forward procedure, there is one important element that must be considered: the individual’s blood type. When combined, two systems, the ABO system and the Rh system (also called the Rhesus system), determine an individual’s blood type.
It is the red blood cells that are involved in defining the blood type. These are the cells responsible for transporting oxygen around the body. On the surface of these cells are molecules known as antigens, which act as markers to the body’s immune system. There are many different types of antigen and it is the difference between these antigens that give rise to the different blood types. The antigens on your own red blood cells are recognised as self-antigens, indicating that the cells they are attached to belong to your body. When the body detects foreign antigens (antigens that aren’t recognised as belonging to you) the immune system attacks these cells and breaks them down. This means that if a person was given blood with the wrong antigens (and therefore the wrong blood type), the immune system would attack the new blood cells and break them down. If this was to happen, not only would the new blood not be useful to the body, it could cause a huge immune response which could ultimately be fatal.
The antigens displayed on your red blood cells depends on your genetic inheritance. This means that your antigens and hence, your blood type, is predetermined by sections of DNA code, known as genes, that you inherit from your parents. The ABO system has A and B antigens; A antigens have one type of sugar chain on their end whilst B antigens have a different sugar chain. You can have either A antigens, B antigens, both (AB) or neither (O) on your red blood cells.
When considering the Rh system it is the D antigen that is important in blood type. The red blood cells either have it (+) or don’t (-). O rhD negative blood (O-) is a fairly rare blood type but it is extremely useful as is does not have the A, B or D antigens and as a result it can be given to anyone. If you are O- then you are a ‘universal donor’. This illustrates the golden rule of blood types: You can receive blood that either has the same antigens as your natural blood or that is missing antigens that your natural blood has, but you can’t receive blood that has extra antigens to your natural blood.
Below is a table that illustrates the 8 blood types using the ABO and the Rh systems.
Ever since we have increased our understanding of blood types, scientists have been trying to find a way to strip the antigens from red blood cells creating an antigen-free blood. This blood, like O- blood, would be able to be given to anyone without the risk of an immune response. The first indication that this might one day be possible occurred in the 1980s when it was found that an enzyme in green coffee beans was able to remove B antigens from red blood cells. Enzymes are biological molecules that act in reactions to help convert one thing into another, in this case they act to help remove the antigens. This investigation was taken one step further in 2007 when enzymes were discovered that could convert A, B and AB blood types into O blood types. This blood was safely transfused into individuals and no immune reaction was elicited. The only problem was that the enzyme was very inefficient and could not be used to produce large amounts of antigen-free blood.
In the past month, scientists have used a technique called directed evolution to genetically engineer a new highly-efficient enzyme from the inefficient enzyme. The enzyme they altered is naturally produced by the bacteria Streptococcus pneumonia, which is often harmlessly carried by healthy individuals but can also cause pneumonia and bacterial meningitis. The researchers mutated the section of bacterial DNA that is used to produce the enzyme. A number of mutations were made in the bacterial DNA so that the bacteria would produce a variety of slightly different new enzymes. From this group of altered bacteria they then selected those that produced the most efficient enzymes and repeated the mutation process. Each time they selected the enzyme most efficient at removing the antigens to develop further. After five generations of mutations they produced an enzyme with 170 times more efficiency than previous. This new enzyme even managed to remove some subtypes of the antigens that the original enzyme found difficult to remove. However, it is not quite able to remove all the antigens, leaving behind residual amounts that could still elicit an immune response. Thus the goal of antigen-free blood has not quite been achieved but is hopefully within reach.
Ultimately, the development of an enzyme that can convert any blood type into being antigen-free could reduce the waiting time to receive blood following trauma and decrease the risk of transfusion. It would also relieve some of the pressure on the NHS to advertise blood shortage crises and recruit individuals of specific blood types, particularly the universal blood donor type.