Chlorophyllin (CHL) was earlier shown to reduce the level of intracellular ROS and apoptosis induced by ionizing radiation and 2,2'-azobis(2-propionimidinedihydrochloride) (AAPH). In the present studies, the effect of CHL on radiation-induced immunosuppression and modulation of immune responses in mice was examined. Chlorophyllin inhibited the in vitro lymphocyte proliferation induced by concanavalin A (Con A) in a dose dependent manner at doses>or=50 microM. At lower doses (10 microM) CHL significantly inhibited activation induced cell death (AICD) in Con A stimulated spleen cells. Spleen cells obtained from CHL treated mice showed an inhibition of response to Con A depending on dose of CHL and the time after its administration. Spleen cells obtained from CHL treated mice (24 h) showed lower inhibition of response to Con A following in vitro (5 Gy) as well as whole body irradiation (2 Gy). The expression of antiapoptotic genes bcl-2 and bcl-xL was up-regulated in these cells. Chlorophyllin treatment of mice led to splenomegaly and increase in the number of peritoneal exudate cells (PEC). The numbers of T cells, B cells and macrophages in the spleen were also increased. Increased phagocytic activity was seen in PEC obtained from CHL treated mice. Most importantly, CHL administration to mice immunized with sheep red blood cells (SRBC) augmented both humoral and cell-mediated immune responses.