Bone Marrow

Tucked inside the hollow centers of your larger bones is a soft tissue called bone marrow. It may not look like much, but it is one of the busiest places in the body. Every day, marrow produces billions of new blood cells: red cells that carry oxygen to your tissues, white cells that defend against infection, and platelets that allow your blood to clot when you are injured.

There are two main types. Red marrow is where blood cells are actively made, and it is rich with the stem cells that give rise to every kind of blood cell. Yellow marrow is made up largely of fat and serves as a reserve. As you age, more of your marrow shifts from red to yellow, though the body can call yellow marrow back into blood-making service when there is a greater need.

Because Bone Marrow divides so rapidly, it is sensitive to treatments that target fast-growing cells, such as chemotherapy. This is why blood counts are watched closely during cancer treatment — when marrow is temporarily affected, levels of red cells, white cells, or platelets can dip. Marrow usually recovers, and care teams have many ways to support you through those changes. Marrow is also central to certain blood cancers and to transplants that can rebuild a healthy blood-making system.

Bone marrow keeps your whole body supplied with the blood cells you depend on every moment — for energy, for immune defense, and for healing. When something affects the marrow, whether a disease or a treatment, the ripple effects can be felt throughout the body, which is why understanding it helps make sense of blood test results and treatment plans.

If you are facing chemotherapy, knowing that fatigue or a higher infection risk may come from temporary effects on the marrow can make those experiences less alarming and easier to discuss with your care team. Marrow's remarkable ability to regenerate is also a source of hope: it is at the heart of treatments that can restore healthy blood production after serious illness.