The complement system is a cascade of enzymes which cooperate in the defense against infections. Many complement proteins are zufinden in serum as inactive enzyme precursors (zymogens); others are located on cell surfaces. The complement system bridges the innate and acquired immunity by enhancing the antibody (Ab) responses and immunologic memory lysis foreign cells Clearin immune complexes and apoptotic cells, the complement components meet numerous biological functions (eg. B. Stimulation of chemotaxis, release of the mast cell degranulation independently from IgE). Complement: There are 3 pathways of complement (pathways of complement): Classic lectin alternative pathways of complement, the classical, the lectin, and the alternative path open last, in the same way when the C3 convertase (C3 con) C3 into C3a and C3b splits. Ab = antibodies; Ag = Antigen; C1-INH = C1 inhibitor, MAC = membrane attack complex; MASP = MBL-associated serine protease; MBL = mannose-binding lectin. Horizontal line shows the activation. The components of the classical pathway are – starting from the order in which they were identified – marked with a C and a number (for. Example, C1, C3). Components of the alternative pathway or provided with a name (eg. B. properdin) often with a letter (D factor z. B. Factor B,). The activation of the classical pathway is Ak-dependent, on the one and occurs when C1 interacts with Ag-IgM or aggregated Ag-IgG complexes; or else it is Ak-independent and occurs when polyanions rule (z. B. heparin, protamine, and RNA DNA of apoptotic cells), gram negative bacteria or bound C-reactive protein (CRP) directly with C1. This pathway is regulated by the C1 inhibitor (C1-INH). Hereditary angioedema occurs due to a genetic deficiency of C1-INH. The activation by the lectin pathway is Ak-independent; it arises due to the binding of mannose-binding lectin (MBL), a serum protein, mannose, fructose, or N-acetylglucosamine groups on bacterial cell walls, yeast cell or virus. Otherwise, this path is similar structurally and functionally the traditional way. The activation of the alternative pathway is initiated when components of microbial cell surface (eg. B. yeast cell walls, lipopolysaccharide [endotoxin] of the bacterial cell walls) or Ig (z. B. nephritischer factor aggregated IgA) cleave small amounts of C3. This path is by properdin, Factor H and DAF (decay accelerating factor, CD55) regulated. The three activation pathways lead last in a common way when the C3 convertase C3 into C3a and C3b splits (pathways of complement). The C3 cleavage can be used to form the membrane attack complex (MAC), the cytotoxic component of the complement system lead. MAK causes the lysis of foreign cells. Factor I with cofactors, including membrane cofactor protein (CD46), disabled C3b and C4b. Patients with a deficiency of complement components C1, C2, C3, MBL, MASP-2, Factor H, Factor I, or complement receptor 2 (CR2) are susceptible to recurrent bacterial infections. Shortages of C5, C9, Factor B, D or properdin factor specifically associated with susceptibility to Neisseria infections. Defects in C1, C4 and C5 are associated with SLE; Defects in CR2 associated with ordinary variable immunodeficiency; and defects in CR3 associated with the type 1 Leukocyte adhesion. Mutations in the genes for Factor B, Factor H, Factor I, membrane cofactor protein (CD46) or C3 have been associated with the development of atypical variant of the hemolytic-uremic syndrome. Biological activities complement perform other immune functions. These are mediated by complement receptors (CR) are located on different cells. CR1 (CD35) supports the phagocytosis and helps in the dissolution of immune complexes. CR2 (CD21) regulates the Ak production by B cells and is the receptor of Epstein-Barr virus. CR3 (CD11b / CD18), CR4 (CD11c / CD18) and C1q receptors play a role in phagocytosis. C3a, C5a and C4a (weak) have anaphylatoxin: They cause the degranulation of Mastzelllen, leading to increased vascular permeability and smooth muscle contraction. C3b acts as opsonin by coating of the microorganisms and the resulting support of phagocytosis. C3d enhances the antibody production by B cells. C5 is a chemical attractant, which regulates the activity of neutrophils and monocytes and enhance adherence of cells, degranulation and release initiate intracellular enzymes from granulocytes, leading to the production of toxic oxygen metabolites and can cause other metabolic events in the cells.