C3b is the larger of two elements formed by the cleavage of complement component 3, and is considered an important part of the innate immune system. C3b is potent in opsonization: tagging pathogens, immune complexes (antigen-antibody), and apoptotic cells for phagocytosis.

C3：血浆中的C3可自然地、缓慢地裂解，持续产生少量的C3b，释入液相中的C3b迅速被I因子灭活。

C3b、C4b和iC3b与病原微生物等非特异性结合，通过与吞噬细胞表面CR1和CR3结合而促进吞噬细胞对病原体的吞噬作用，称为补体介导的调理作用。 三、炎症介导作用

在人体中，C3蛋白不断“自发”低水平地被分解为两种较小的蛋白质C3a和C3b。其中C3b较为活泼，其可与两个常见的化学基团氨基或羟基结合。所以外来和非自体表面上的碳水化合物、脂质和蛋白质很容易被C3b附着。

C3是血清中含量最高的补体成分，主要由巨噬细胞和肝脏合成，在C3转化酶的作用下，裂解成C3a和C3b两个片段，在补体经典激活途径和旁路激活途径中均发挥重要作用。

2017年10月26日 · 补体裂解产物C3b与细菌或其他颗粒结合，可促进吞噬细胞的吞噬，称为补体的调理作用。

2006年10月15日 · Here we present a structure of C3b that reveals a marked loss of secondary structure in the CUB (for ‘complement C1r/C1s, Uegf, Bmp1’) domain, which together with the resulting translocation ...

C3b is a highly unstable molecule that has a unique thioester that allows covalent binding to a microbial cell. Some C3b fragments bind to the target cell, where they act as opsonins. In the classic pathway, most C3b is found in an AbC4bC2aC3b complex on the cell surface, which is called the C5 convertase .

2021年5月12日 · 由这种自发切割产生的蛋白质片段之一——C3b，具有很强的反应性，并能和两种常见的化学基团（氨基或羟基）结合。由于许多构成“入侵者”表面（如细菌的细胞壁）的蛋白质和糖类都有氨基或羟基，因此C3b“手榴弹”具有许多的靶点。

C3b binds covalently through its thioester bond to adjacent molecules on the pathogen surface; otherwise it is inactivated by hydrolysis. C3 is the most abundant complement protein in plasma, occurring at a concentration of 1.2 mg ml –1 , and up to 1000 molecules of C3b can bind in the vicinity of a single active C3 convertase (see Fig. 2.11 ).