Clotting and immune defense in Limulidae

The blue blood of the horseshoe crab contains a sophisticated defense system very sensitive to pathogens or foreign materials. The hemocytes circulating in the hemolymph detect trace amounts of LPS molecules on the invading microorganisms and respond quickly to release the granular components into the external milieu. The coagulation system composed of three serine protease zymogens, factor C, factor B, and proclotting enzyme, and a clottable protein, coagulogen, is activated by LPS to form insoluble coagulin gel. The coagulation system also responds to beta-(1,3) glucan through the activation of unique heterodimeric serine protease zymogen, factor G. The pathogens are, thus, engulfed in the gel and subsequently killed by antimicrobial substances with various specificities, which are also released from cells. The horseshoe crab has developed two kinds of serine protease zymogens as biological sensors, factor C and factor G, which are responsive to LPS and beta-(1,3) glucan on the surface of Gram-negative bacteria and fungi, respectively. These are possible invaders for horseshoe crabs and also for most animals including humans. This novel heterodimeric serine protease zymogen, factor G, may open a new way to develop an innovative assay system to quantitate beta-(1,3) glucans. Furthermore, these LPS and beta-(1,3) glucan sensitive factors could be utilized as a unique tool to analyze other biological reactions caused by LPS or the glucan. Although the coagulation reaction in horseshoe crabs is famous, it is not the only defense mechanism of this animal. Many agglutinins are present either in hemolymph plasma or in the cell. The hemolymph plasma also has cytolytic activity against foreign cells. These cellular and humoral defense systems, in concert, defend themselves from invading foreign organisms. Such a sophisticated defense system has allowed the horseshoe crab to survive for more than 200 million years on the earth. Horseshoe crabs are often called ¿living fossils." However, they are not fossils. They are living.