Initial triggering of complement

Complement activation is due to a cascade of proteolytic steps, performed by serine protease domains in some of the components. Three different pathways of activation are distinguished triggered by target-bound antibody (the classical pathway); microbial polysaccharide structures (the lectin pathway); or recognition of other "foreign" surface structures (the alternative pathway) by C3b. All three merge in the pivotal activation of C3 and, subsequently, of C5 by highly specific enzymatic complexes, the so-called C3/C5 convertases. A complement system with three C3 activation pathways and a common lytic pathway is found only in jawed vertebrates.

补体激活源于一系列蛋白酶解步骤的级联作用，这些步骤由某些组分中的丝氨酸蛋白酶结构域执行。根据靶标结合抗体（经典途径）、微生物多糖结构（凝集素途径）或C3b对其他“异源”表面结构的识别（替代途径），可区分出三种不同的激活途径。这三种途径最终汇聚于C3的临界激活，进而通过高度特异性的酶复合物，即所谓的C3/C5转化酶，激活C5。仅存在于有颌脊椎动物中的补体系统，具有三条C3激活途径和一条共同的溶解途径。