<P> The complement system is a biochemical cascade that attacks the surfaces of foreign cells . It contains over 20 different proteins and is named for its ability to "complement" the killing of pathogens by antibodies . Complement is the major humoral component of the innate immune response . Many species have complement systems, including non-mammals like plants, fish, and some invertebrates . </P> <P> In humans, this response is activated by complement binding to antibodies that have attached to these microbes or the binding of complement proteins to carbohydrates on the surfaces of microbes . This recognition signal triggers a rapid killing response . The speed of the response is a result of signal amplification that occurs after sequential proteolytic activation of complement molecules, which are also proteases . After complement proteins initially bind to the microbe, they activate their protease activity, which in turn activates other complement proteases, and so on . This produces a catalytic cascade that amplifies the initial signal by controlled positive feedback . The cascade results in the production of peptides that attract immune cells, increase vascular permeability, and opsonize (coat) the surface of a pathogen, marking it for destruction . This deposition of complement can also kill cells directly by disrupting their plasma membrane . </P> <P> Leukocytes (white blood cells) act like independent, single - celled organisms and are the second arm of the innate immune system . The innate leukocytes include the phagocytes (macrophages, neutrophils, and dendritic cells), innate lymphoid cells, mast cells, eosinophils, basophils, and natural killer cells . These cells identify and eliminate pathogens, either by attacking larger pathogens through contact or by engulfing and then killing microorganisms . Innate cells are also important mediators in lymphoid organ development and the activation of the adaptive immune system . </P> <P> Phagocytosis is an important feature of cellular innate immunity performed by cells called phagocytes that engulf, or eat, pathogens or particles . Phagocytes generally patrol the body searching for pathogens, but can be called to specific locations by cytokines . Once a pathogen has been engulfed by a phagocyte, it becomes trapped in an intracellular vesicle called a phagosome, which subsequently fuses with another vesicle called a lysosome to form a phagolysosome . The pathogen is killed by the activity of digestive enzymes or following a respiratory burst that releases free radicals into the phagolysosome . Phagocytosis evolved as a means of acquiring nutrients, but this role was extended in phagocytes to include engulfment of pathogens as a defense mechanism . Phagocytosis probably represents the oldest form of host defense, as phagocytes have been identified in both vertebrate and invertebrate animals . </P>

How many cells does the immune system have