Q: Which blood group is rarest?

Among the eight common groups, AB is generally rarest, and negative groups are generally rarer than positive ones.

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What blood groups actually are, how they're inherited, how they're tested, and why some are rarer than others — the biology behind the letters and signs on a donor card.

Almost everyone has heard the terms A, B, AB, and O, along with a plus or minus sign. But what determines your blood group, and why do some show up far more often than others?

This guide covers the biology behind blood groups — for how different groups relate to each other during a transfusion, see Understanding Blood Group Compatibility.

What Determines Your Blood Group?

Your blood group is determined by specific molecules called antigens on the surface of your red blood cells. The two systems used in everyday blood typing are:

ABO system — based on whether your red cells carry A antigen, B antigen, both, or neither.
Rh system — based mainly on whether the RhD antigen is present (the + or − sign).

Combining the two gives the eight common blood groups: O+, O−, A+, A−, B+, B−, AB+, and AB−.

How Blood Group Is Inherited

Blood group is genetically inherited — you receive one ABO gene copy from each parent, and the combination determines your blood group. This is also why blood group can't change based on lifestyle, diet, or anything you do — it's set at conception and generally stays fixed for life.

A common misconception: a child's blood group must "match" one of their parents' exactly. In reality, depending on the specific gene combinations involved, a child can end up with a blood group that looks different from either parent's on the surface — this is normal genetics, not a red flag.

How Blood Group Is Tested

Blood typing is done in a lab using a small blood sample. Specific reagents are mixed with the sample — if red cells clump together (agglutinate) in reaction to a particular reagent, that antigen is present. This is how both the ABO group and the Rh factor are confirmed.

You don't need to know your blood group in advance to become a donor — it's determined as part of the donation process itself.

How Common Is Each Blood Group?

Blood group distribution varies by country and population, so there's no single universal percentage that applies everywhere. That said, some broad patterns hold across most populations:

O and A are generally the most common ABO groups.
AB is generally the rarest ABO group.
Rh-positive is generally far more common than Rh-negative in most populations, including in India.

Because negative blood groups are less common, and O-negative in particular is often needed in emergencies (see our compatibility guide), donors with negative blood groups are especially valuable — though every blood group is genuinely needed to keep supply steady.

Rare and Special Blood Types

Beyond the eight common groups, there are much rarer blood types recognized by more specialized antigen systems. One notable example with roots in India is the Bombay blood group (also called Oh), first identified in Mumbai in 1952. People with this extremely rare type lack antigens that even O-type blood carries, meaning they can generally only safely receive blood from other Bombay-phenotype donors — not even standard O-negative blood is compatible for them.

This is exactly why maintaining a broad, diverse donor base matters — some patients depend on donor types that are far rarer than the standard eight groups.

Beyond ABO and Rh

ABO and Rh are the two systems used in routine blood typing and everyday transfusion matching, but they aren't the only blood group systems that exist — researchers have identified dozens of others, most of which rarely affect standard transfusions but can matter for patients who need repeated transfusions or highly specific matching.

Does Everyone Have a Blood Group?

Yes — every person has an ABO group and an Rh type, whether or not they've ever been tested. If you've never donated blood or had a lab test that included blood typing, you may simply not know yours yet — and that's completely fine.

Frequently Asked Questions

Can my blood group change over time?

No, not under normal circumstances. Blood group is genetically determined at conception and stays fixed for life, aside from rare and unusual medical situations.

Can a child have a different blood group from both parents?

Yes — depending on the specific gene combinations each parent carries, a child's blood group can look different from either parent's on the surface. This is normal genetics.

Which blood group is rarest?

Among the eight common groups, AB is generally the rarest, and negative groups are generally rarer than positive ones. Beyond the common groups, rare types like the Bombay blood group are far rarer still.

Do I need to know my blood group before donating?

No — it's determined as part of the donation process through lab testing.

What is the Bombay blood group?

An extremely rare blood type, first identified in Mumbai, where red cells lack antigens present even in standard O-type blood — meaning compatible donors are very limited.

Is one blood group "better" than another?

No. Every blood group plays a necessary role in keeping blood supply available for the patients who need it.

Medical & Safety Disclaimer

This article is provided by Lifia for general educational purposes and is not medical advice. Population statistics referenced here are general patterns, not precise figures for any specific region, and individual blood typing should always be confirmed through proper laboratory testing.

Sources & Further Reading

Directorate General of Health Services (DGHS), Ministry of Health & Family Welfare, Government of India — Transfusion Medicine Technical Manual and Blood Transfusion Services guidance.
National Blood Transfusion Council (NBTC), Government of India.

Content review: September 2026

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