Fucoidan, a dietary fiber contained in seaweed, reduces the increase of antigen-specific IgE in mice exposed to ovalbumin. In this we investigated the effect of fucoidan on IgE production and intracellular events in B cells in vitro. Fucoidan inhibited the production of IeE and Cc germline transcription in murine B cells induced by IL-4 (100 ng/ml) and anti-CD40 antibodies (10 mu g/ml), whereas it stimulated cell proliferation. A significant effect of fucoidan on IgE production was observed when B cells were stimulated with a higher dose (5 mu g/ml) of anti-CD40 antibodies, but not when stimulated with lower doses (1.25, 2.5 mu g/ml), regardless of the IL-4 concentrations. Moreover, nuclear translocation of NF kappa B p52, but neither that of NF kappa B p65, nor the phosphorylation of JAK1 and STAT6 was reduced by fucoidan. These results suggest that fucoidan inhibited IgE production by preventing the NF kappa B p52-mediated pathways activated by CD40. (c) 2006 Elsevier Inc. All rights reserved.