There seem to be two different mechanisms responsible for CH reduction by PUVA. Local suppression by topical PUVA treatment was thought to be a result of blocking the afferent phase of immune response, it was associated with a lack of CH effector cells in the peripheral lymph nodes and could not be reversed by indomethacin treatment. Diffuse suppression induced by systemic PUVA treatment seemed to be associated with blocking of egress of effector cells from the regional lymph nodes, this depressed CH response was prevented when indomethacin was administered before PUVA treatment.