Mycosis fungoides and the Sézary syndrome are diseases in which lymphocytes (a type of white blood cell) become malignant (cancerous) and affect the skin.
Normally, the bone marrow makes blood stem cells (immature cells) that become mature blood stem cells over time. A blood stem cell may become a myeloid stem cell or a lymphoid stem cell. A myeloid stem cell becomes a red blood cell, white blood cell, or platelet. A lymphoid stem cell becomes a lymphoblast and then one of three types of lymphocytes (white blood cells):
- B-cell lymphocytes that make antibodies to help fight infection.
- T-cell lymphocytes that help B-lymphocytes make the antibodies that help fight infection.
- Natural killer cells that attack cancer cells and viruses.
Blood cell development. A blood stem cell goes through several steps to become a red blood cell, platelet, or white blood cell.
In mycosis fungoides, T-cell lymphocytes become cancerous and affect the skin. In the Sézary syndrome, cancerous T-cell lymphocytes affect the skin and are in the blood.
Mycosis fungoides and the Sézary syndrome are types of cutaneous T-cell lymphoma.
Mycosis fungoides and the Sézary syndrome are the two most common types of cutaneous T-cell lymphoma (a type of non-Hodgkin lymphoma). For information about other types of skin cancer or non-Hodgkin lymphoma, see the following PDQ summaries:
A sign of mycosis fungoides is a red rash on the skin.
Mycosis fungoides may go through the following phases:
- Premycotic phase: A scaly, red rash in areas of the body that usually are not exposed to the sun. This rash does not cause symptoms and may last for months or years. It is hard to diagnose the rash as mycosis fungoides during this phase.
- Patch phase: Thin, reddened, eczema -like rash.
- Plaque phase: Small raised bumps (papules) or hardened lesions on the skin, which may be reddened.
- Tumor phase: Tumors form on the skin. These tumors may develop ulcers and the skin may get infected.
Check with your doctor if you have any of these signs.