Your body is made up of lots of types of cells. T cells travel around in your blood and are known as lymphocytes. They are a part of your immune system. One of their roles is to help your body fight off diseases or infections caused by bacteria and viruses.

Sometimes, the body makes too many of a certain type of cell that are different (abnormal) compared to the healthy version; this is known as cancer. Lymphoma is a cancer of the lymphocytes, or white blood cells, in this case T cells specifically. T cells are a part of your immune system. So, when too many T cells are made, this is known as T-cell lymphoma.

The first signs of CTCL appear on the skin where cancerous T cells build up and often look like a rash. It is important to understand that CTCL is a blood cancer, even though it first affects your skin.

There are several types of CTCL that can look different on the skin, affect different parts of the body and grow at different speeds. The most well known types are called mycosis fungoides (MF) and Sézary syndrome (SS).

Experts do not know what causes CTCL. However, scientific studies have shown that it is not contagious (this means that you cannot catch it from other people or pass it on) and generally not genetic (this means that it is unlikely that you will pass it on to your children).