Mycosis fungoides is a skin disease caused by cells from your immune system collecting in the skin. These cells are called T-lymphocytes and are one type of the white cells in your blood that help fight infection. Many of these lymphocytes in your skin are abnormal and their presence causes the changes that you have noticed.
Mycosis Fungoides is the most common form of Cutaneous T-cell Lymphoma. Cutaneous Lymphoma is however rare, affecting 6 per million people per year
In most people, these abnormal lymphocytes only build up in the skin. Other parts of the body are only affected in rare cases.