People have different blood types because inherited genes influence the markers displayed on the surface of their red blood cells. In the ABO system, these markers are sugars: type A cells carry A antigen, type B carry B, type AB carry both, and type O has neither A nor B. The Rh system adds another important marker, commonly described as positive or negative.
Blood cells carry identifying markers
Red blood cells have many surface molecules called antigens. The immune system can recognize these molecules. In the ABO system, A and B antigens are sugar structures on the cell surface. Genes inherited from a person’s parents influence which enzymes build those structures. [1]
How the familiar ABO groups differ
People with type A blood have A antigen, people with type B have B, and people with AB have both. Type O cells have neither A nor B. These group names describe a small part of a much larger set of blood-group systems; scientists have identified many other red-cell antigens too. [2]
Why compatibility matters
People can have antibodies that react against ABO antigens they do not have. If incompatible red cells are transfused, the immune system may attack them, which can cause a serious reaction. That is why medical teams test blood and follow compatibility procedures before transfusion. The simple labels A, B, AB and O are useful, but they do not capture every factor clinicians consider.
What does the plus or minus sign mean?
The positive or negative sign usually refers to the Rh(D) antigen. If the D antigen is present, the type is Rh-positive; if absent, it is Rh-negative. Blood type is inherited, and it is not a measure of health, personality or strength.
Sources and evidence
- NCBI Bookshelf — Blood Groups and Red Cell Antigens: Antigens on red blood cells ↗
NIH-hosted reference explains red-cell antigens and how genes determine ABO sugar markers.
- NCBI Bookshelf — The ABO blood group ↗
Reference details ABO antigens, inheritance and variation among populations.
Sources consulted: 15 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; the explanation was checked against the linked institutional and research sources.