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Indolent vs Advanced Types of Systemic Mastocytosis

What people with indolent systemic mastocytosis (ISM) should know about the more advanced forms of the disorder.

A senior woman speaks with her hematologist about the results of blood tests.

Updated on May 27, 2024

Mast cells are a type of white blood cell that are located in many different tissues and organs throughout the body. Like other white blood cells, mast cells help the body fight infections and respond to injury. More specifically, mast cells release chemicals (called mediators) that promote inflammation, recruit other immune cells, and promote the healing of damaged tissue (among other immune functions). Mast cells are also what release histamine, a proinflammatory mediator associated with allergies.

Mastocytosis is a group of disorders where the body produces abnormal mast cells. The abnormal mast cells have a genetic mutation that causes them to proliferate (grow and divide) more quickly than normal mast cells.

As a result, excess numbers of mast cells accumulate in different parts of the body. As these mast cells release mediators, inflammation can damage surrounding tissues and disrupt how organs function. Large amounts of mediators being released can also result in anaphylaxis, a life-threatening allergic reaction that requires immediate treatment with an epinephrine injection.

Mastocytosis that only affects the skin is called cutaneous mastocytosis. Mastocytosis that affects organs beyond the skin is referred to as systemic mastocytosis. Systemic mastocytosis can affect the skin, bone marrow, gastrointestinal tract, liver, lymph nodes, and the central nervous system.

Systemic mastocytosis can further be categorized into less aggressive and more aggressive subtypes.

Indolent systemic mastocytosis

When used to describe disorders like systemic mastocytosis, the term “indolent” means that the disease is progressing slowly. Indolent systemic mastocytosis (ISM) is the least severe form of the disorder, and also the most common. It tends to cause skin symptoms and can affect multiple organs, but the level of abnormal mast cells is low enough that it does not disrupt the functioning of the organs that are affected.

However, indolent systemic mastocytosis can cause symptoms, including fatigue, fainting, gastrointestinal distress, headaches, and congestion. People with this type usually have a normal life expectancy, though the disease does progress to more advanced types in up to 5 percent of cases.

Smoldering systemic mastocytosis

While not categorized as an advanced form of systemic mastocytosis, smoldering systemic mastocytosis is more aggressive than ISM. This means there are higher numbers of abnormal mast cells, symptoms will be more severe, and there is a greater risk of progression to an advanced type.

Advanced types of systemic mastocytosis

The following types of systemic mastocytosis are considered advanced. These types progress more quickly, causing damage to organs and a loss of organ function.

Aggressive systemic mastocytosis

With this type, the number of abnormal mast cells is high enough to significantly interfere with the functioning of major tissues and organs. This includes the bone marrow, liver, small intestine, spleen, lymph nodes, and bones. This type can progress quickly and cause severe symptoms.

Systemic mastocytosis with an associated hematologic neoplasm

With this type, systemic mastocytosis occurs alongside another disease that impairs the production of normal blood cells. Hematologic refers to diseases that affect the blood and/or bone marrow, and neoplasm refers to an abnormal growth inside of the body.

Mast cell leukemia

Leukemias are a group of cancers that originate in the bone marrow, when tissues that produce blood cells produce abnormal blood cells. Mast cell leukemia occurs when the bone marrow produces large numbers of abnormal immature mast cells. This type is rare compared to the other types of systemic mastocytosis, but it is the most aggressive type.

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