The innate immune system is an immediate, nonspecific response to harmful substances. It is composed of physical barriers, such as the skin, as well as chemical and microbiological barriers, including the mucous secretions of the respiratory tract and the normal microflora of the gastrointestinal tract. Primary components of the innate immune system include monocytes, macrophages, and neutrophils—immune cells that engulf and digest invading microorganisms in a process called phagocytosis. These cells express surface receptors that identify pattern-recognition molecules that are conserved among several families of pathogens but unique to pathogenic microorganisms. A key component of innate immunity is the complement system, a biochemical cascade of at least 20 serum proteins that functions to kill invading pathogens by direct lysis (cell rupture) or through the promotion of phagocytosis. Innate immunity also involves cells that mediate the inflammatory response. Thus, the innate immune response is a rapid, nonspecific response that serves as the first line of defense against invading micro-organisms. However, if the innate response doesn’t adequately eliminate microorganisms and resultant infections, the adaptive immune system is summoned to action through the signaling of messenger proteins called cytokines.