Created by: CK-12/Adapted by Christine Miller

Like the little girl shown in Figure 6.1.1, seven-year-old Mateo is battling leukemia, a type of cancer that affects blood cells. Leukemia usually starts in the bone marrow where blood cells are produced. It causes the production of abnormal blood cells, most commonly white blood cells. Depending on the type of leukemia, it can also affect other types of blood cells. The abnormal blood cells replace the patient’s normal blood cells over time, which can lead to symptoms of fatigue, frequent infections, and easy bruising or bleeding. Leukemia can be fatal, but fortunately, there are some treatment options available that can prolong life — and may even cure the disease.

Mateo has undergone chemotherapy to kill the cancerous cells, but his doctors have told his parents that it is not enough. Mateo needs a bone marrow transplant in order to replace his abnormal bone marrow with healthy bone marrow. His family members are eager to donate bone marrow to him, but first they must be tested to see if they are a compatible match.

For blood transfusions, it is relatively easy to find a compatible blood donor, but bone marrow transplants require much more specific matching between donor and recipient. They must share several of the same type of proteins — called human leukocyte antigens (HLAs) — on the surface of their cells. One type of HLA protein is illustrated in Figure 6.1.3. Different people have different types of HLA proteins (or markers) depending on their specific genes. Typically, eight to ten HLA markers are tested and compared in the potential bone marrow donor and recipient. At least six or seven of these HLA markers must be identical between them in order for a match to be made.

If the match is not good, the patient’s body could reject the bone marrow transplant. Conversely, the transplanted bone marrow could produce immune cells that attack the patient’s body. A good match between donor and recipient is critical for bone marrow donation to be safe and effective.

A full sibling frequently provides the best match for bone marrow donation because they share many of the same genes from their parents. Mateo’s sister is tested, but unfortunately, she is not a match for him. This is not all that surprising since there is only about a 25 per cent chance that a sibling will be an identical HLA match. His parents and other family members are also tested, but none of them are a match, either. Mateo must join the 70 per cent of patients that need to look outside of their families for a bone marrow donor.

How do you find a bone marrow match outside of your family? Fortunately, people from all over the world have signed up to be potential bone marrow donors, usually by providing a simple swab of the inside of their cheek. DNA from the cells collected on the swab is then tested for HLA type. The potential donor’s HLA information is put into a donor registry, and doctors can then search national and international registries for compatible matches for their patients.

Patients are much more likely to be a match with a bone marrow donor of their same race or ethnic background. People with similar ancestry are more likely to share similar HLA genes. In Mateo’s case, his mother is African American, and his father is Japanese and Caucasian. His relatively rare combination of ethnic backgrounds may make it harder for him to find a match in the donor registries, as is the case for many multiethnic patients.

Read the rest of this chapter to learn more about the genetic and phenotypic variations that exist in humans, and how some of these differences came about due to differing natural selection pressures in different areas of the world. At the end of the chapter, learn more about Mateo’s quest for a bone marrow donor, the need for bone marrow donors from diverse ethnic backgrounds, and how you may be able to save someone’s life based on your genetic makeup!