photopheresis

noun

pho·​to·​phe·​re·​sis -fə-ˈrē-səs 

: an immunomodulating therapy used especially to treat cutaneous T-cell lymphomas that involves treating blood with a photoactive drug (as methoxsalen), obtaining a fraction rich in white blood cells, exposing the fraction to damaging ultraviolet radiation, and returning it to the body where it stimulates a therapeutic immunological response