Blood Group Serology
Blood groups for Group A, B, O, and AB,(commonly termed ABO) and Rhesus (Rh) are the two main blood group systems that are screened for every unit of blood collected. Using fully automated instruments with microplate agglutination testing, the ABO, Rh and irregular red cell antibodies typing are confirmed for every donation before transfusion.
Rhesus Blood Group
The Rhesus blood group is the 2nd most significant blood group system after the ABO blood group. It consists of 5 main antigens, the most important being the "D" antigen. This is responsible for the "+" and "-" that we often see next to our ABO blood group, eg. A+, B+, O+, AB+, A-, B-, O-, AB-.
In Singapore, most people are Rhesus positive (A+, B+, O+, AB+), and only less than 1% of the population are Rhesus negative (A-, B-, O- and AB-). This Rhesus-negative blood type is found most commonly in Singaporean Indians (6%), compared to other racial groups (less than 1%).
Rhesus-negative blood is considered not only rare but also precious because people who are Rhesus-negative can only receive blood that is also Rhesus-negative.
Bombay Blood Group
In addition to the well-known ABO and Rhesus blood groups, there are in fact many other lesser known but nonetheless important blood groups in humans. One such example is the Bombay blood group.
If Rhesus negative blood is considered rare because it is found about 1 in 1,000 persons, you can understand how much rarer the Bombay blood group is because it is found in only one in 10,000 Indians and almost 1 in 1,000 000 Europeans.
The reason behind the Bombay blood group being so rare is because individuals with this blood group do not express the "H antigen" (which is needed to make the red cells of blood type O). These individuals also cannot also make the "A" and "B" antigens (which give rise to the A and B blood types), because the "H" antigen is the "starting block" for them to be made. Those with Bombay blood group cannot receive blood type A, B, O or AB. They can only receive from those with Bombay blood type.