Extracorporeal photopheresis (ECP) is a therapy currently used to treat some types of cancer, including cutaneous T-cell lymphoma (CTCL), a rare malignancy of the lymphatic tissue that involves the abnormal proliferation of T lymphocytes in the skin. This therapy consists in taking lymphocytes which, after being treated with photo responsive drugs activated by UVA light, are reinfused into the patient where they generate a response from the immune system which consequently regulates itself to kill cancer cells. Although the ECP has been a technology applied for more than 25 years to treat different kinds of diseases, the mode of action of the therapy remains still unknown and it is thought to be mainly attributable to an immunomodulatory effect that originates when the treated lymphocytes are reintroduced inside the patient.