The mechanism of ECP beyond the effects of apoptosis induction is incompletely understood. Studies to date suggest that ECP in autoimmune conditions or GVHD stimulates anti-idiotypic responses against host tissue-reactive T cell clones, attenuates antigen-presenting function by type 2 dendritic cells, and induces anti-inflammatory cytokine responses. In addition, an important component of tolerance induction is thought to involve the expansion of Tregs that suppress alloreactive mechanisms.