This is an observational study on extracorporeal photopheresis,an established therapy for the treatment of Graft versus Host Disease (GvHD) post allogeneic haemopoietic stem cell transplantation. There are different techniques and devices to perform cell collection procedure, photoactivation and infusion of mononuclear cells.

The investigators will enrol patients undergoing allogeneic transplants and extracorporeal photopheresis in order to understand whether the different ways in which photopheresis is performed affect cell products and clinical response.

The FPG Stem Cell Transplantation Programme has indicated extracorporeal photopheresis as the therapy of choice for the second-line treatment of steroid-resistant GvHD. If efficacy is supported by clear evidence in the literature, less clear are the mechanism of action, the qualitative and quantitative characteristics of the cells responsible for the clinical response, and the impact of procedural and instrumental variables on clinical efficacy. Indeed, there are different techniques and devices to perform cell collection procedure, photoactivation and infusion of mononuclear cells.

The investigators designed a prospective study that aims to collect data on patients, photopheresis procedures and related cellular products. The analysis of the data will allow the investigators to define the qualitative and quantitative characteristics of the cell products, associating them with the different techniques and instruments, and looking for possible associations with clinical response.

The data collected will be able to contribute to the characterisation of the techniques, procedures and the definition of a minimum cell dose to be treated in order to obtain the response. The aim is to define the volume to be processed or the number of procedures to be carried out by analogy with what is now established practice for the collection procedures of other autologous or allogeneic therapeutic cells (stem cells, lymphocytes, granulocytes), laying the foundations for optimising the treatments and resources employed.

Inclusion Criteria:

• diagnosis of GvHD post allogeneic HSC transplantation being treated with photopheresis;
• signing of informed consent (IC) and consent to personal data processing

Exclusion Criteria:

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The study population is represented by patients affected by chronic or acute GVHD after allogeneic haematopoietic stem cell transplantation treated at the 'Haematology' and 'Laboratory and DH of Haematology' units undergoing photopheretic treatment in Apheresis Unit of the Fondazione Policlinico Universitario Agostino Gemelli (FPG) - IRCCS.