Blood grouping is one of the most important safety steps in blood transfusion. To understand why matching is so critical, it helps to know the basics of the ABO and Rh systems.

The ABO blood group system

The ABO system classifies blood into four main groups:

·       Group A

·       Group B

·       Group AB

·       Group O

The group is determined by specific markers (antigens) on the surface of red blood cells:

·       Group A: A antigens on red cells, anti‑B antibodies in plasma.

·       Group B: B antigens on red cells, anti‑A antibodies in plasma.

·       Group AB: A and B antigens on red cells, no anti‑A or anti‑B antibodies.

·       Group O: No A or B antigens on red cells, both anti‑A and anti‑B antibodies in plasma.

These antibodies are naturally present and will attack red cells that carry the corresponding antigen. For example, a person with group A has anti‑B antibodies. If they receive group B red cells, their antibodies can destroy those cells, causing a dangerous reaction.

The Rh (Rhesus) system

In addition to ABO, the Rh system is important. The main Rh antigen is called D:

·       If you have the D antigen, you are Rh positive (+).

·       If you do not have it, you are Rh negative (−).

So your full blood type might be A+, O−, AB+, and so on. Rh matching matters because an Rh‑negative person can develop antibodies if exposed to Rh‑positive blood. Once those antibodies form, future transfusions or even future pregnancies can be at risk if Rh‑positive red cells are present.

This is especially important for women of child‑bearing age. If an Rh‑negative woman develops anti‑D antibodies and later carries an Rh‑positive baby, those antibodies can attack the baby’s red cells. To prevent this, doctors try to give Rh‑negative women Rh‑negative blood whenever possible and may use special preventive injections in pregnancy.