system
Given the following post, respond as if you're commenting on Reddit:
human
What makes you a compatible bone marrow donor?
gpt
Blood cancer survivor here!

Your body is made up of tiny factories called cells and inside each cell is a really big molecule called DNA.  Each DNA molecule tells the cell that contains it what to do and each DNA molecule contains all the information needed to build you... all of you.

DNA is so big that your body stores it like an encyclopedia, in volumes.  Scientists call those volumes "Chromosomes."  On Chromosome number six is a special part of your DNA called the HLA.  The HLA stands for "Human Leukocyte Antigen."  

The HLA part of your DNA tells your body what the outside of your cells should "look" like.  When your body is building a new cell it uses molecules called proteins to make the outside of the cell match to what the HLA says it should look like.  

When you get sick your body sends White Blood Cells -- sort of the Marines of your immune system -- after the germs that make you sick.  White Blood Cells are big and powerful cells and it would be bad if they beat up on parts of your body that aren't sick.  They look at the outside of cells and leave the ones that match your HLA alone.  That way they only beat up cells that don't belong... cells that don't have your DNA.

Sometimes when a person gets **SUPER** sick they need a bone marrow transplant.  Bone marrow is a special cellular factory deep inside your bones where your body makes new blood cells including white blood cells.  When a person's bone marrow is replaced they get a WHOLE NEW FACTORY but that factory comes with all of the instructions -- all of the DNA -- that made it work in the person who donated it.  

That means that it comes with the donors HLA and that the white blood cells it produces will attack and kill cells that don't match the donor's HLA.

So if the donor's HLA doesn't match the recipients HLA the donated bone marrow will make white blood cells which will try to kill the recipient.  Doctors call that "graft vs host disease."

That's why it's so hard to find bone marrow donors.  You need to find someone who has a nearly identical HLA.  You need to fool the donated cells into thinking that they're in the body they came out of; that means you need an almost identical genetic match.  

Those are hard to find.  Sometimes even brothers and sisters aren't a close enough match.  Fortunately, there are billions of people all over the world and while it might be a million to one shot of finding a donor, we have six-thousand-million people to choose from!