Fucoidin is a polymer of predominantly sulfated L-fucose found in Fucus vesiculosis (Bladderwrack).
Fucoidan significantly inhibits tight binding of human sperm to human zona pellucida in vitro. 
It is a significant inhibitor of leukocyte adhesion, exacerbates acute ischemic renal failure and stimulates nitric oxide synthesis. 
The fucoidan found in bladderwrack inhibits the growth of many unfriendly bacteria and viruses. Some of the viruses this compound is antagonistic to include herpes simplex virus, human cytomegalovirus, and human immunodeficiency virus. Bladderwrack has been found to agglutinate the cells of several strains of Candida. Bladderwrack also has a toxic effect on some strains of E. coli and all strains tested of Neisseria meningitidis.
Fucoidin (FU) and Arabinoglactan (AG) appear to be synergistic immunomodulators. Macrophages treated with AG and FU (10-100 microg/mL) exhibited induced tumoricidal activity and increased phagocytosis, lysosomal enzyme activity, and production of nitrite, H2O2, tumor necrosis factor (TNF)-alpha, and interleukin (IL)-6. However, AG and FU had little effect on the level of IL-1beta. Thus, the tumoricidal effect of AG- and FU-activated macrophages appeared to be mainly mediated by production of free radicals (NO and H2O2) and cytokines (TNF-alpha and IL-6). These data suggest that AG and FU are activators of lymphocytes and macrophages. This property may contribute to their effectiveness in the immunoprevention of cancer.
A revised structure for fucoidan may explain some of its biological activities
J Biol Chem 1993 Oct 15;268(29):21770-21776 Patankar MS, Oehninger S, Barnett T, Williams RL, Clark GF
Fucoidan from Fucus vesiculosus inhibits human sperm-zona pellucida binding and blocks the zona pellucida-induced acrosome reaction in human sperm. Fucoidan also potently inhibits selectins-mediated adhesion of leukocytes to vascular endothelium. To understand the molecular basis for fucoidan's inhibition of specific cell adhesion events, we have investigated the structure of this fucan using definitive methods of carbohydrate structural analysis.