The present invention relates in essence to a method for detecting a subset of cells with a binding domain which is specific for a target which characterizes said subset of cells, which method comprises detecting, ex vivo, said subset of cells, wherein said binding domain is to be administered to a subject which comprises or is assumed to comprise said subset of cells, prior to the removal of said subset of cells. The present invention also relates to the use of a binding domain as defined herein in a method defined herein. The use of a binding domain as defined herein for the preparation of a diagnostic composition to be used in a method defined herein is also envisaged. In another aspect, the present invention relates to the binding domain as defined herein to be used in a method defined herein. Kits comprising a binding domain as defined herein and means to administer said binding domain to a subject, and, optionally, means to detect said binding domain with a method defined herein, are also disclosed. In another aspect, the present invention relates to the use of a binding domain, preferably a therapeutically effective antibody like for example alemtuzumab, apolizumab, cetuximab, epratuzumab, galiximab, gemtuzumab, ipilimumab, labetuzumab, panitumumab, rituximab, trastuzumab, nimotuzumab, mapatumumab, matuzumab, rhMab ICR62, rhMab B-Ly1 and pertuzumab etc including combinations thereof, for the preparation of a pharmaceutical composition for the treatment of patients disposed to respond favorably to said binding domain as identified by a method defined herein.La presente invención es concerniente en esencia con un método para detectar u subconjunto de células con un dominio de enlace que es especifico para un objetivo que caracteriza dicho subconjunto de células, el método comprende detectar ex vivo dicho subconjunto de células, en donde dicho dominio de enlace va a ser administrado a un sujeto que comprende o se supone que comprende dicho subconjunto de células
