What is HLA — and why does it matter?

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HLA stands for human leukocyte antigen. Think of it as a set of markers sitting on the surface of your cells — a kind of biological barcode that tells your immune system, "this belongs to me." You inherit this barcode from your parents.

In a transplant, the donor's barcode needs to be close enough to the patient's. If it isn't, two problems can follow: the patient's body may reject the donor cells, or — more importantly — the donor's new immune system may see the patient's body as foreign and attack it, the reaction called graft-versus-host disease.

That's why matching is the foundation of a donor transplant. A test called HLA typing, done from a simple blood sample, reads the barcode of the patient and each possible donor so the team can compare them. You can read more about the reaction matching helps prevent in our guide to graft-versus-host disease →

Why a brother or sister is only a one-in-four chance

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Many families are surprised to learn a sibling isn't automatically a match. Here's the simple reason.

Each of us inherits one half of our HLA barcode from our mother and one half from our father. Any child gets one of two possible halves from each parent. When you work through the combinations, two full siblings have roughly a one-in-four chance of inheriting the very same pair — a full match. There's about a one-in-two chance of a half match, and a one-in-four chance of no useful match at all.

So a matched sibling is wonderful when it happens, but it's a coin-toss of genetics, not a given — which is exactly why other routes exist.

When there's no matched sibling — the options

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If no sibling matches, the search doesn't stop — it just moves to the next option. There are three, and a good transplant team works through them in order of what suits the patient best.

A half-matched family donor (haploidentical)

Because a parent is always a half match — and children and half-matched siblings often are too — a family member can usually step in. Techniques for half-matched transplants have advanced enormously, and units in India are especially experienced with them. This is why almost every patient now has a possible donor within their own family.

A matched unrelated donor from a registry

Millions of people worldwide have volunteered to donate stem cells, and their HLA types are held in donor registries — such as DATRI in India, and international registries like DKMS and Be The Match. A search checks these databases for a matched stranger. It's a genuine gift from a volunteer, entirely voluntary and unpaid.

Umbilical cord blood

Banked cord blood is another source. Cord units don't need to match as tightly, which helps when nothing else is found, and they're used mostly in children because a single unit holds a limited number of cells.

A general guide. Which route suits a patient depends on the condition, the urgency, and what donors are available. For how these map to different transplant types, see types of bone marrow transplant →

How does a donor actually give cells?

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People often picture something painful, but for most donors it's straightforward. In the most common method, the donor has a few days of injections that coax stem cells out of the marrow and into the bloodstream, then gives cells through a vein in a process similar to donating blood platelets — no surgery.

The other method, a bone marrow harvest, collects cells directly from the hip bone under anaesthetic — a short procedure, after which the donor's own marrow replaces what was taken within a few weeks. Either way, donating is safe for a healthy donor, and the medical team looks after them carefully.

How long does the search take?

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If a family donor is available, matching is quick — the relatives are simply tested, and a transplant can be planned without delay. That speed is one reason haploidentical transplants are so valuable when time matters.

A registry search takes longer — typically weeks to a few months — because a matched volunteer has to be found, confirmed, and their cells collected and sometimes shipped from another country. This is also why an unrelated-donor transplant costs more. If speed is important, a specialist may lean toward a family donor for that reason alone.

Common questions

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