title: Complement: activation, consequences, and control.
abstract: The activation of complement provides the humoral (fluid-phase) effector mechanism most responsible for immune-mediated injury. The classical pathway is activated by an antigen-antibody reaction. The binding of C1q initiates the sequential activation of the eleven proteins. The classical pathway has a calcium-dependent step (C1q, C1r, C1s) and a magnesium-dependent reaction (the enzymatic action of C1s on C4 and C2). The alternative pathway appears to be spontaneously activated, but the perpetuation of that activation is dependent upon the availability of an activating (or protective) surface which interferes with the inactivation of C3b by control proteins. The alternative pathway has a magnesium-dependent step, the binding of B to C3b to form the C3 convertase. Once initiated, the alternative pathway activation results in the sequential activation of nine proteins, six of which are common to both pathways. The activation of complement results in a variety of biologic consequences which can result in injury to the host. The potential destructiveness of the effects of complement activation is modulated by a series of control proteins.
mesh_terms: D002352:Carrier Proteins / Q000494:pharmacology; D003167:Complement Activation; D003174:Complement C1 Inactivator Proteins / Q000494:pharmacology; D003176:Complement C3 / Q000378:metabolism; D003178:Complement C3 Nephritic Factor / Q000494:pharmacology; D003180:Complement C3b Inactivator Proteins / Q000494:pharmacology; D003182:Complement C5 / Q000378:metabolism; D017242:Complement Factor H; D003169:Complement Inactivator Proteins; D015938:Complement Membrane Attack Complex; D003170:Complement Pathway, Alternative; D003171:Complement Pathway, Classical; D003165:Complement System Proteins / Q000378:metabolism; D006461:Hemolysis; D006801:Humans; D016167:Integrin alphaXbeta2; D011414:Properdin / Q000494:pharmacology
chemical_list: C493495:CFH protein, human; D002352:Carrier Proteins; D003174:Complement C1 Inactivator Proteins; D003176:Complement C3; D003178:Complement C3 Nephritic Factor; D003180:Complement C3b Inactivator Proteins; D003182:Complement C5; D003169:Complement Inactivator Proteins; D015938:Complement Membrane Attack Complex; D016167:Integrin alphaXbeta2; D011414:Properdin; D017242:Complement Factor H; D003165:Complement System Proteins