The innate immune system, also known as non-specific immune system and first line of defense, is a subsystem of the overall immune system that comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner. Innate immune systems provide immediate defense against infection, and are found in all classes of plant and animal life. They include both humoral immunity components and cell-mediated immunity components. The major functions of the vertebrate innate immune system include recruiting immune cells to sites of infection, activation of the complement cascade, identification and removal of foreign substances present, activation of the adaptive immune system, and acting as a physical and chemical barrier to infectious agents.