Myelofibrosis (MF) is a rare blood disorder that affects your bone marrow. It’s a form of chronic leukemia and about 18,000 people in the United States live with myelofibrosis. Most are over 60 years old, but the disorder affects younger people as well. MF does affect your bone marrow, but the condition also affects other parts of your body, such as your spleen. When this happens, your treatment may not be as effective. How Does Myelofibrosis Affect the Spleen? Normally, blood cells are formed in your bone marrow, but if you have myelofibrosis, scar tissue starts to form inside the marrow and this displaces the normal blood-forming tissue. As a result, you can’t produce enough blood cells to meet your needs and your body responds by trying to produce more blood cells in your liver and spleen. Since producing blood cells is not the organs’ usual role, the extra work makes them enlarge. This may lead to other complications. Not everyone who has myelofibrosis develops an enlarged spleen, but it is a fairly common complication. What Does the Spleen Do? Your spleen is a fist-sized organ in your abdomen, to the left and below your rib cage side. Although it has several roles, your spleen’s most important job is to help boost your immune system. It filters and recycles your blood cells and platelets, and acts as a storage place. Your spleen can become damaged by trauma, such as blow to your abdomen from a car accident, or illness or infection. A spleen will not enlarge on its own, so if you do have enlarged spleen, a condition called splenomegaly, your doctor will likely refer you for testing to try to discover the cause. It is possible to live without a spleen, but without a spleen, you may contract infections more easily and your body will have a harder time fighting them off. What Does Splenomegaly Do to the Body? As your spleen gets larger, it could cause a number of complications, such as: Bleeding: Platelets are blood cells that help your blood to clot. An enlarged spleen can deplete what platelets you have left in your circulation, leaving you vulnerable should you start to bleed. Infections: White blood cells help fight infection. Just as the spleen can deplete the number of platelets, it could use up your white blood cells, causing a drop to the point where your body will have a hard time fighting off infections. Anemia: Your red blood cells carry iron, which is one of the most important nutrients in your blood. An enlarged spleen may destroy the red blood cells, causing you to develop anemia. Tissue death: If your spleen becomes large enough, it may cut off its own blood supply, causing tissue in the spleen to die. Pain: If the spleen is enlarged, it may press against the organs around it, causing pain or discomfort. Risk of rupture: As the spleen grows, it becomes more delicate and a blow or injury to the area could cause it to rupture. These potential complications can make it hard for your doctor to treat your myelofibrosis. What Is the Treatment for an Enlarged Spleen Caused by Myelofibrosis? Researchers are still working on finding the best way to treat an enlarged spleen caused by myelofibrosis. Right now, treatment depends on how it affects you. If you have no symptoms, your hematologist may choose to “watch and wait,” to see if your spleen will get larger or if you will develop symptoms. But if you are having symptoms, your doctor may choose to try biologic medications, such as ruxolitinib. If medications don’t work, your doctor may recommend that you have a splenectomy, removal of your spleen. It is possible to have myelofibrosis for many years without having symptoms. If your spleen does start to grow, your hematologist would likely be able to feel it when pressing on your abdomen during a physical examination.