The human organism responds to injury with complex predetermined patterns that, at a tissue level, have their analogues in lower animals. In animal phyla, the first responses to injury to evolve were phagocytosis and regeneration (present in amebas, hydras, sponges, etc). Phagocytosis, which at the level of these organisms is the engulfment of a solid particle by a cell, involves only simple recognition of damage or of status as foreign versus self. A more advanced level of response occurs in larger multicellular animals (invertebrates such as worms, mollusks, insects), in which the existence of a vascular system permits mobilization and transport of specialized inflammatory cells (phagocytes) to the site of injury. This nonspecific acute inflammatory response goes beyond simple recognition and phagocytosis to include chemotaxis (movement of cells in response to a chemical concentration gradient) and microcirculatory changes. In vertebrates, a highly specific immune response exists that enhances the efficiency of phagocytosis and the acute inflammatory response to injury. This enhancement is possible because of the presence of cells (lymphocytes) that remember an encounter with an injurious agent and produce a greater, more specific, and faster response when they meet that particular agent again. Specificity, memory, and amplification are the trio of features that distinguish the immune response from the acute inflammatory reaction.