Know Your Blood Group and Rhesus Type

Most people know that blood comes in different types, but far fewer know their own. Your blood group is a simple, one-off thing to establish, and it can matter at important moments — in pregnancy, before surgery, and when giving or receiving blood. A quick blood test settles it for life.

The ABO system

Your ABO group describes which markers, called antigens, sit on the surface of your red blood cells. There are four main groups:

• Group A — A antigens on the cells.
• Group B — B antigens.
• Group AB — both A and B antigens.
• Group O — neither A nor B.

These groups determine which blood types are compatible during a transfusion, which is why matching is so carefully checked in hospital.

The Rhesus (D) factor

On top of your ABO group, you are either Rhesus D positive or negative, depending on whether a further marker — the RhD antigen — is present. This is where the plus or minus in a type like "O positive" or "A negative" comes from. Around the majority of people are RhD positive, with a minority RhD negative.

Why it matters

Blood group and Rhesus status come into play in several situations:

• Pregnancy — an RhD-negative mother carrying an RhD-positive baby may need care to prevent a reaction; this is routinely managed in antenatal care.
• Transfusion — giving compatible blood is essential, so your group is always confirmed before a transfusion.
• Donation — knowing your group is a small step towards becoming a blood donor, and some groups are in especially high demand.

How the test works

Establishing your blood group needs nothing more than a standard blood sample. In the laboratory, your red cells are tested against known reagents to see which antigens are present, revealing both your ABO group and your Rhesus status. It is a well-established, routine test, and because your group does not change, you only ever need it done once.

When to get checked

There is no medical urgency to knowing your blood group day to day, but it can be reassuring and practical to have it on record — particularly if you are planning a pregnancy, considering blood donation, or simply like to know your own health details. If your circumstances mean it is clinically important, such as in pregnancy, your maternity team will arrange the necessary testing as part of routine care.

Written by Dr Ibrar Ahmed — MBChB MRCP MRCGP PhD PG Dip (Cardiology)
Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic