HUMORAL IMMUNITY IN LONG-LIVED ARTHROPODS

The innate immune system is represented by a complex of cellular and h umoral factors and processes that protect the organism against pathoge nic attack. Certain of these elements arose early in the evolution of multicellular animals and have been preserved throughout the evolution ary divergence of the different animal phyla. Two mediators that have interested us are alpha(2)-macroglobulin and the pentraxins. Both func tion as elements in the plasma defense systems of arthropods and verte brates. The ability to purify significant quantities of both proteins from the plasma of the American horseshoe crab Limulus polyphemus has enabled us to investigate their functions at the molecular and organis mal levels. In Limulus alpha(2)-macroglobulin functions as a broad-spe ctrum protease-binding molecule that mediates the clearance of proteas es from the plasma. Limulus has at least two pentraxins in the plasma, an abundant form designated Limulus C-reactive protein, which is of u nknown function, and limulin, a low-abundance form with sialic acid re cognition capabilities, which mediates a cytolytic reactivity for the destruction of foreign cells that have contacted the plasma.