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At any given moment, trillions of cells 
are traveling through your blood vessels,

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sometimes circling the body 
in just one minute.

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Each of these cells 
has its origins deep in your bones.

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Bones might seem rock-solid, 
but they’re actually quite porous inside.

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Large and small blood vessels 
enter through these holes.

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And inside most of the large bones 
of your skeleton is a hollow core

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filled with soft bone marrow.

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Marrow contains fat 
and other supportive tissue,

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but its most essential elements 
are blood stem cells.

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These stem cells are constantly dividing.

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They can differentiate 
into red blood cells,

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white blood cells, and platelets,

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and send about hundreds of billions 
of new blood cells

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into circulation every day.

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These new cells enter the bloodstream

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through holes 
in small capillaries in the marrow.

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Through the capillaries,

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they reach larger blood vessels 
and exit the bone.

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If there’s a problem with your blood,

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there’s a good chance 
it can be traced back to the bone marrow.

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Blood cancers often begin 
with genetic mutations in the stem cells.

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The stem cells themselves 
are not cancerous,

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but these mutations can interfere 
with the process of differentiation

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and result in malignant blood cells.

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So for patients with advanced 
blood cancers like leukemia and lymphoma,

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the best chance for a cure is often 
an allogeneic bone marrow transplant,

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which replaces the patient’s bone marrow 
with a donor’s.

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Here’s how it works.

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First, blood stem cells 
are extracted from the donor.

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Most commonly,

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blood stem cells are filtered out 
of the donor’s bloodstream

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by circulating the blood 
through a machine

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that separates it 
into different components.

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In other cases, 
the marrow is extracted directly

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from a bone in the hip, the iliac crest, 
with a needle.

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Meanwhile, the recipient 
prepares for the transplant.

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High doses of chemotherapy or radiation 
kill the patient’s existing marrow,

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destroying both malignant cells 
and blood stem cells.

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This also weakens the immune system,

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making it less likely 
to attack the transplanted cells.

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Then the donor cells are infused into 
the patient’s body through a central line.

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They initially circulate 
in the recipient’s peripheral bloodstream,

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but molecules on the stem cells, 
called chemokines, act as homing devices

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and quickly traffic them 
back to the marrow.

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Over the course of a few weeks,

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they begin to multiply and start producing
new, healthy blood cells.

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Just a small population 
of blood stem cells

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can regenerate a whole body’s 
worth of healthy marrow.

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A bone marrow transplant 
can also lead to something

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called graft-versus-tumor activity,

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when new immune cells 
generated by the donated marrow

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can wipe out cancer cells the recipient’s 
original immune system couldn’t.

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This phenomenon can help eradicate 
stubborn blood cancers.

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But bone marrow transplants 
also come with risks,

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including graft-versus-host disease.

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It happens when the immune system 
generated by the donor cells

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attacks the patient’s organs.

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This life-threatening condition 
occurs in about 30–50% of patients

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who receive donor cells 
from anyone other than an identical twin,

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particularly when the stem cells 
are collected

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from the blood 
as opposed to the bone marrow.

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Patients may take 
immunosuppressant medications

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or certain immune cells may be removed 
from the donated sample

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in order to reduce the risk 
of graft-versus-host disease.

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But even if a patient 
avoids graft-versus-host disease,

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their immune system 
may reject the donor cells.

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So it’s crucial to find the best match 
possible in the first place.

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Key regions of the genetic code 
determine how the immune system

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identifies foreign cells.

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If these regions are similar 
in the donor and the recipient,

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the recipient’s immune system 
is more likely to accept the donor cells.

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Because these genes are inherited, 
the best matches are often siblings.

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But many patients 
who need a bone marrow transplant

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don’t have a matched family member.

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Those patients 
turn to donor registries of volunteers

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willing to offer their bone marrow.

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All it takes to be on the registry is 
a cheek swab to test for a genetic match.

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And in many cases, 
the donation itself

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isn’t much more complicated 
than giving blood.

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It’s a way to save someone’s life

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with a resource 
that’s completely renewable.