How bones make blood

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

sometimes circling the body
in just one minute.

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

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

Large and small blood vessels
enter through these holes.

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

filled with soft bone marrow.

Marrow contains fat
and other supportive tissue,

but its most essential elements
are blood stem cells.

These stem cells are constantly dividing.

They can differentiate
into red blood cells,

white blood cells, and platelets,

and send about hundreds of billions
of new blood cells

into circulation every day.

These new cells enter the bloodstream

through holes
in small capillaries in the marrow.

Through the capillaries,

they reach larger blood vessels
and exit the bone.

If there’s a problem with your blood,

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

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

The stem cells themselves
are not cancerous,

but these mutations can interfere
with the process of differentiation

and result in malignant blood cells.

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

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

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

Here’s how it works.

First, blood stem cells
are extracted from the donor.

Most commonly,

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

by circulating the blood
through a machine

that separates it
into different components.

In other cases,
the marrow is extracted directly

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

Meanwhile, the recipient
prepares for the transplant.

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

destroying both malignant cells
and blood stem cells.

This also weakens the immune system,

making it less likely
to attack the transplanted cells.

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

They initially circulate
in the recipient’s peripheral bloodstream,

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

and quickly traffic them
back to the marrow.

Over the course of a few weeks,

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

Just a small population
of blood stem cells

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

A bone marrow transplant
can also lead to something

called graft-versus-tumor activity,

when new immune cells
generated by the donated marrow

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

This phenomenon can help eradicate
stubborn blood cancers.

But bone marrow transplants
also come with risks,

including graft-versus-host disease.

It happens when the immune system
generated by the donor cells

attacks the patient’s organs.

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

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

particularly when the stem cells
are collected

from the blood
as opposed to the bone marrow.

Patients may take
immunosuppressant medications

or certain immune cells may be removed
from the donated sample

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

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

their immune system
may reject the donor cells.

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

Key regions of the genetic code
determine how the immune system

identifies foreign cells.

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

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

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

But many patients
who need a bone marrow transplant

don’t have a matched family member.

Those patients
turn to donor registries of volunteers

willing to offer their bone marrow.
And in many cases,
the donation itself

isn’t much more complicated
than giving blood.

It’s a way to save someone’s life

with a resource
that’s completely renewable.

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