Blood type is determined by markers on red blood cells. The ABO system defines four main groups: A, B, AB, and O. The Rh factor adds a positive (+) or negative (–) label (for example, A+ or O−). These labels are used to match donors and recipients to prevent dangerous transfusion reactions.
Compatibility simplified
People with O negative blood are universal donors for red cells (can be given to almost anyone), while AB positive people are universal recipients. However, component transfusions (plasma, platelets) have different compatibility rules. Hospitals follow strict matching and cross-matching procedures before transfusion.
Why this matters for donors
Your blood type affects who your donation can help. Blood services often target drives to recruit types that are in short supply. If you know your blood type, consider donating when your type is needed — your blood could be particularly valuable.
How types are determined
Blood typing is done in the lab by mixing a small sample with reagents that react to A, B and Rh markers. Donors are informed if they don't already know their type, which can be helpful for future donations and medical records.
This overview is informational. For clinical questions about compatibility or transfusion, consult your regional blood service or clinical team.