About Myelofibrosis & the Spleen

What does your spleen do?

Graphic that shows where the spleen is located in the body

The spleen is a small organ located near the stomach under your left rib cage. Among its many important functions, the spleen helps your body with:

  • Filtering the blood
  • Storing blood cells
  • Fighting infection

The spleen helps the body filter the blood

Healthy red blood cells (A) are able to pass back into circulation through small holes or slits in veins (B) in the spleen, while old or damaged cells (C) cannot and are broken down.

Images, including enlarged spleen above, are shown for illustrative purposes.

Cross-section of Spleen

Graphic showing cross-section of the spleen

Watch Myelofibrosis, Spleen Size & You

Take a closer look at the various functions of the spleen and learn more about why spleen size is an important health focus for people with MF.

What causes splenomegaly (an enlarged spleen) in MF ?

Typically, the spleen is about the size of your fist. However, in people with myelofibrosis (MF), it may become enlarged.

In people with MF, scar tissue builds up within the bone marrow, preventing it from making enough normal blood cells. Red blood cell and platelet counts typically decrease over time, while white blood cell counts may increase or decrease. The spleen partially takes over blood cell production, and this may make the spleen get bigger—a condition called splenomegaly (splee-nuh-MEG-uh-lee). This is a common finding in people with MF.

An enlarged spleen can be a sign that your spleen is working harder than normal. However, in some patients with MF, it may also be a sign of disease progression, which means that your MF is changing or getting worse.


Three graphics showing – first- Normal blood cell production, 2nd In, MF, scar tissue decreases blood cell production, 3rd Spleen gets larger as it helps to produce blood cells

In Myelofibrosis

Three graphics showing – first- Normal blood cell production, 2nd In, MF, scar tissue decreases blood cell production, 3rd Spleen gets larger as it helps to produce blood cells
Three graphics showing – first- Normal blood cell production, 2nd In, MF, scar tissue decreases blood cell production, 3rd Spleen gets larger as it helps to produce blood cells
Icon of 90% in a circle

In one clinical study, about 90% of people living with MF had an enlarged spleen at diagnosis.

What are the symptoms of an enlarged spleen?

Because the spleen is located in your abdominal area, an enlarged spleen may press on the stomach, making it difficult for the stomach to expand normally when eating. Symptoms of an enlarged spleen may include:

  • Abdominal discomfort
  • Pain under the left ribs
  • An early feeling of fullness when eating (early satiety)—even if you haven’t eaten much food
Icon of two intertwined speech bubbles

It’s important to remember to talk to your Healthcare Professional about all of your symptoms, even if you're not sure they’re related to your MF.

How will my Healthcare Professional check my spleen size?

It’s important to work with your Healthcare Professional to establish your “baseline” spleen size at diagnosis—or as early on as possible. Your Healthcare Professional can check your spleen size by gently feeling or “palpating” your left upper abdominal area. Because it can sometimes be difficult to accurately measure spleen size through physical examination alone, your Healthcare Professional may order an imaging test. These tests may include:

Icon of an ultrasound


Icon of a CT scan

Computed tomography (CT),
also called a CT scan

Icon of an MRI

Magnetic resonance imaging (MRI)

Early identification of an enlarged spleen is important and may help your Healthcare Professional to reassess your care management approach sooner if your MF is progressing. Remember, it’s important to work with your care team to regularly keep track of any changes in your spleen size or symptoms.

Icon of a spiral notepad

Know 5 key questions to help prepare for your next MF care visit

Recognizing the signs and symptoms of spleen enlargement can help you take a more active role in your ongoing MF care. Learn what questions you should be asking yourself before your next MF care visit.

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Jakafi is a prescription medicine used to treat adults with polycythemia vera who have already taken a medicine called hydroxyurea and it did not work well enough or they could not tolerate it.

Jakafi is used to treat adults with certain types of myelofibrosis.

Jakafi is also used to treat adults and children 12 years of age and older with acute graft-versus-host disease (GVHD) who have taken corticosteroids and they did not work well enough.

Important Safety Information

Jakafi can cause serious side effects, including:

Low blood counts: Jakafi® (ruxolitinib) may cause your platelet, red blood cell, or white blood cell counts to be lowered. If you develop bleeding, stop taking Jakafi and call your healthcare provider. Your healthcare provider will perform blood tests to check your blood counts before you start Jakafi and regularly during your treatment. Your healthcare provider may change your dose of Jakafi or stop your treatment based on the results of your blood tests. Tell your healthcare provider right away if you develop or have worsening symptoms such as unusual bleeding, bruising, tiredness, shortness of breath, or a fever.

Infection: You may be at risk for developing a serious infection during treatment with Jakafi. Tell your healthcare provider if you develop any of the following symptoms of infection: chills, nausea, vomiting, aches, weakness, fever, painful skin rash or blisters.

Skin cancers: Some people who take Jakafi have developed certain types of non-melanoma skin cancers. Tell your healthcare provider if you develop any new or changing skin lesions.

Increases in cholesterol: You may have changes in your blood cholesterol levels. Your healthcare provider will do blood tests to check your cholesterol levels during your treatment with Jakafi.

The most common side effects of Jakafi include: for certain types of MF and PV – low platelet or red blood cell counts, bruising, dizziness, headache, and diarrhea; and for acute GVHD – low platelet, red or white blood cell counts, infections, and fluid retention.

These are not all the possible side effects of Jakafi. Ask your pharmacist or healthcare provider for more information. Tell your healthcare provider about any side effect that bothers you or that does not go away.

Before taking Jakafi, tell your healthcare provider about: all the medications, vitamins, and herbal supplements you are taking and all your medical conditions, including if you have an infection, have or had tuberculosis (TB) or have been in close contact with someone who has TB, have or had hepatitis B, have or had liver or kidney problems, are on dialysis, have high cholesterol or triglycerides, had skin cancer, or have any other medical condition. Take Jakafi exactly as your healthcare provider tells you. Do not change your dose or stop taking Jakafi without first talking to your healthcare provider.

Women should not take Jakafi while pregnant or planning to become pregnant. Do not breastfeed during treatment with Jakafi and for 2 weeks after the final dose.

Please see the Full Prescribing Information, which includes a more complete discussion of the risks associated with Jakafi.

You are encouraged to report negative side effects of prescription drugs to the FDA. Visit, or call 1-800-FDA-1088.

You may also report side effects to Incyte Medical Information at 1-855-463-3463.