Quick answer
O negative (O−) means red blood cells carry no A antigen, no B antigen, and no RhD antigen. Because these red cells lack the antigens most likely to trigger a reaction, O− is often called the universal red cell donor. O− people can generally receive red cells only from other O− donors.
What O− means
O indicates no A or B antigens; the minus sign indicates no RhD antigen. With none of these three major antigens present, O− red cells are generally compatible with all eight common recipient types.
O− plasma, by contrast, contains both anti-A and anti-B antibodies and is generally compatible only with type O recipients.
O− is frequently used in emergencies when a patient's type is not yet known, which is one reason blood services actively recruit O− donors.
| Property | Value |
|---|---|
| ABO group | O |
| Rh (D) status | Negative (RhD antigen absent) |
| Red cell antigens | None (ABO); no RhD |
| Plasma antibodies (typical) | anti-A, anti-B |
Compatibility summary
General red cell and plasma compatibility for O− under the simplified ABO/Rh model. Real transfusions also depend on antibody screening and crossmatching.
| Product | Can give to | Can receive from |
|---|---|---|
| Red blood cells | O−, O+, A−, A+, B−, B+, AB−, AB+ | O− |
| Plasma (ABO only) | O | A, B, AB, O |
See the full compatibility chart for all eight types.
Inheritance context
Type O requires the OO genotype and Rh-negative requires dd. Each parent contributed an O allele and a d allele.
An O− child can have Rh-positive parents (if both are Dd) and A or B parents (if they are AO or BO).
Two O− parents are expected to have only O− children under the standard model, which is the one combination the baby calculator reports as an exact 100%.
Work through specific parent combinations with the baby blood type calculator, or find which parent types could produce a O− child with the parent calculator.
Limitations
- Blood type can only be confirmed by laboratory testing. Family history, calculators, and self-report are not substitutes.
- Compatibility rules on this page are general educational summaries. Real transfusion decisions also depend on antibody screening, crossmatching, product type, and clinical judgment.
- Rare variants such as the Bombay phenotype, cis-AB, and weak or partial D expression can produce results that differ from the simplified ABO/Rh model.