Some blood groups are more common than others. For example, O+ and B+ are generally common in South Asia, while AB− and some Rh‑negative groups are rarer. Even if your group is common, hospitals need a lot of it because many patients share that group.

If you have a rarer group (e.g., O−, AB−), you may be especially valuable because fewer donors can help patients with those types. But every blood group is important; the blood bank must maintain a healthy mix of all types to serve the whole population.