Vivanco F, Muñoz E, Vidarte L, Pastor C
Department of Immunology, Fundación Jimenez Diaz, Madrid, Spain.
Mol Immunol. 1999 Sep-Oct;36(13-14):843-52. doi: 10.1016/s0161-5890(99)00105-4.
Antigens (Ags) are converted into immune complexes (antigen-antibody complexes, IC) as soon as they encounter their specific antibodies (Abs). In fluids containing complement, the process of IC formation and fixation of complement components occur simultaneously. Hence, the formation of Ag-Ab-complement complexes is the normal way of eliminating Ags from a host. C3b-C3b-IgG covalent complexes are immediately formed on interaction of serum C3 with IgG-IC. These C3b-C3b dimers constitute the core for the assembly of C3/C5-convertase on the IC, which are subsequently converted into iC3b-iC3b-IgG by the complement regulators. These complexes are detected on SDS-PAGE by two bands of molecular composition, C3alpha65-C3alpha43 (band A) and C3alpha65-heavy chain of the Ab (band B), which correspond to C3b-C3b and C3b-IgG covalent interaction respectively, and that identify opsonized IC (C3b-IC). C3b can attach to Fab and Fc regions of the Ab molecule with similar efficiency. The presence of multiple C3b binding regions on IgG is considered an advantageous characteristic that facilitates the elimination of Ags in the form of C3b(n)-IC. Ab molecules on the IC recognize the Ag, and also serve as a very good acceptor for C3b binding. In this way, Ags, even if they have no acceptor sites for C3b, can be efficiently processed and removed. When C3 is activated in serum by IC or other activators, secondary C3b-IgG covalent complexes are generated, with bystander monomeric circulating IgG, and thus constitute, physiological products of complement activation. These complexes gain importance when IgG concentration is extremely high as in cases of infusion of intravenous IgG (IVIG) in several pathologies. The covalent attachment of activated complement C3 (C3b, iC3b, C3 d,g) to Ags or IC links innate and adaptative immunity by targeting Ags to different cells of the immune system (follicular dendritic cells, phagocytes, B cells). Hence C3b marks Ags definitively, from the earliest contact with the innate immune system until their complete elimination from the host.
抗原（Ags）一旦遇到其特异性抗体（Abs），就会转化为免疫复合物（抗原 - 抗体复合物，IC）。在含有补体的液体中，IC的形成过程和补体成分的固定过程同时发生。因此，Ag - Ab - 补体复合物的形成是从宿主中清除Ags的正常方式。血清C3与IgG - IC相互作用时会立即形成C3b - C3b - IgG共价复合物。这些C3b - C3b二聚体构成了IC上C3/C5转化酶组装的核心，随后被补体调节因子转化为iC3b - iC3b - IgG。这些复合物在SDS - PAGE上通过两条分子组成带被检测到，即C3α65 - C3α43（带A）和Ab的C3α65 - 重链（带B），它们分别对应于C3b - C3b和C3b - IgG共价相互作用，并且可识别调理素化的IC（C3b - IC）。C3b可以以相似的效率附着于Ab分子的Fab和Fc区域。IgG上多个C3b结合区域的存在被认为是一个有利特征，有助于以C3b(n) - IC的形式清除Ags。IC上的Ab分子识别Ag，并且也是C3b结合的良好受体。通过这种方式，即使Ags没有C3b的受体位点，也可以被有效地处理和清除。当C3在血清中被IC或其他激活剂激活时，会产生二级C3b - IgG共价复合物以及旁观者循环单体IgG，因此构成补体激活的生理产物。当IgG浓度极高时，如在几种疾病中静脉注射IgG（IVIG）的情况下，这些复合物就变得很重要。活化的补体C3（C3b、iC
