Humoral immunity in long-lived arthropods

The innate immune system is represented by a complex of cellular and humoral factors and processes that protect the organism against pathogenic attack. Certain of these elements arose early in the evolution of multicellular animals and have been preserved throughout the evolutionary divergence of the different animal phyla. Two mediators that have interested us are α2-macroglobulin and the pentraxins. Both function as elements in the plasma defense systems of arthropods and vertebrates. 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 organismal levels. In Limulus α2-macroglobulin functions as a broad-spectrum protease-binding molecule that mediates the clearance of proteases from the plasma. Limulus has at least two pentraxins in the plasma, an abundant form designated Limulus C-reactive protein, which is of unknown function, and limulin, a low-abundance form with sialic acid recognition capabilities, which mediates a cytolytic reactivity for the destruction of foreign cells that have contacted the plasma.