The innate, or nonspecific, immune system is one of the two main immunity strategies (the other being the adaptive immune system) in vertebrates. The innate immune system is an alternate defense strategy and is the dominant immune system response found in plants, fungi, insects, and primitive multicellular organisms (see Beyond vertebrates). The major functions of the innate immune system are to: recruit immune cells to infection sites by producing chemical factors, including chemical mediators called cytokines activate the complement cascade to identify bacteria, activate cells, and promote clearance of antibody complexes or dead cells identify and remove foreign substances present in organs, tissues, blood and lymph, by specialized white blood cells activate the adaptive immune system through antigen presentation act as a physical and chemical barrier to infectious agents; via physical measures such as skin and chemical measures such as clotting factors in blood, which are released following a contusion or other injury that breaks through the first-line physical barrier (not to be confused with a second-line physical or chemical barrier, such as the blood–brain barrier, which protects the nervous system from pathogens that have already gained access to the host). Anatomical barriers Anatomical barriers include physical, chemical and biological barriers. The epithelial surfaces form a physical barrier that is impermeable to most infectious agents, acting as the first line of defense against invading organisms. Desquamation (shedding) of skin epithelium also helps remove bacteria and other infectious agents that have adhered to the epithelial surface. Lack of blood vessels, the inability of the epidermis to retain moisture, and the presence of sebaceous glands in the dermis, produces an environment unsuitable for the survival of microbes. In the gastrointestinal and respiratory tract, movement due to peristalsis or cilia, respectively, helps remove infectious agents. Also, mucus traps infectious agents. Gut flora can prevent the colonization of pathogenic bacteria by secreting toxic substances or by competing with pathogenic bacteria for nutrients or cell surface attachment sites. The flushing action of tears and saliva helps prevent infection of the eyes and mouth. Inflammation Inflammation is one of the first responses of the immune system to infection or irritation. Inflammation is stimulated by chemical factors released by injured cells. It establishes a physical barrier against the spread of infection and promotes healing of any damaged tissue following pathogen clearance. The process of acute inflammation is initiated by cells already present in all tissues, mainly resident