The complement proteins are a key component of the innate immune system. The term innate immunity has been coined to differentiate this type of immunity from that termed adaptive immunity.<sup>19 Medzhitov R., Janeway C.A. An ancient system of host defense Curr Opin Immunol 1998 ;  10 : 12-15  [cross-ref]

Cliccare qui per andare alla sezione Riferimenti</sup> In higher organisms, animals respond to the antigenic challenge presented by invading organisms by the development of a specific immune response. Antibody is formed and cells that have the capacity to recognize the specific foreign antigen are generated. Thus, a sophisticated mechanism for defending against infection exists. Nevertheless, more primitive and higher organisms have mechanisms for fighting infection under circumstances in which no specific immunity exists. Animals often rely on such a system when no previous exposure to an organism has occurred and before adaptive immunity comes into play.

Elements of the innate immune system include small peptides, such as defensins, that have the capacity to interact with and destroy organisms<sup>17 Lehrer R.I., Lichtenstein A.K., Ganz T. Defensins: Antimicrobial and cytotoxic peptides of mammalian cells Ann Rev Immunol 1993 ;  11 : 105-128  [cross-ref]

Cliccare qui per andare alla sezione Riferimenti</sup>; receptor systems, such as those on phagocytes, that recognize lipopolysaccharide; sugars on the surface of organisms,<sup>8 Feizi T. Carbohydrate recognition systems in innate immunity Adv Exp Med Biol 1998 ;  435 : 51-54

Cliccare qui per andare alla sezione Riferimenti</sup> such as Toll and CD14, that aid in the phagocytic process; and a host of other factors that interact to provide a measure of defense. The complement proteins are of interest because they seem to be a key component of the innate immune system but have been recruited during the phylogenetic development of adaptive immunity to participate in this more sophisticated immune response also.<sup>2 Carroll M.C., Prodeus A.P. Linkages of innate and adaptive immunity Curr Opin Immunol 1998 ;  10 : 36-40

Cliccare qui per andare alla sezione Riferimenti</sup> Complement proteins consist of circulating glycoproteins, cell-bound receptors for these proteins that provide for effector function, and a host of regulatory molecules, both circulating and cell bound, that control activity of the pathways of activation.<sup>11 The Human Complement System in Health and Disease New York: Marcel Dekker (1998). 

Cliccare qui per andare alla sezione Riferimenti</sup> The reason for the many regulatory molecules is that uncontrolled activation of complement can cause host tissue damage, an unwanted result of microorganism attack. Antibodies, one product of the adaptive immune system, also can activate complement. If these antibodies are directed at host tissue, as in autoimmune disease, uncontrolled complement activation could cause severe tissue damage. Being able to downregulate complement activity is as important as maintaining complement function.