The exact mechanism of action of ECP is unknown, but the principle of the process is to induce leucocyte apoptosis with UVA radiation after their exposure to psoralens, which are light sensitizers. These leukocytes are immediately re-infused into the patient, where they undergo early apoptosis. Following apoptosis, the leukocytes are engulfed by macrophage or other antigen-presenting cells, such as immature dendritic cells, in an anti-inflammatory cytokine environment. The anti-inflammatory cytokine secretion pattern, with a switch from TH1 to TH2 for CD4+ lymphocytes, and the engulfment by immature cells without co-stimulatory molecules induces anergy, by deleting effector T-cells that responded to the presented antigens. An increase in regulatory T-cells (T-regs) is also induced after ECP and may contribute to allograft acceptance by the recipient.