Complement activation, classical pathway

The complement system is a biochemical cascade that helps, or complements, the ability of antibodies to clear pathogens from an organism. It is part of the immune system called the innate immune system that is not adaptable and does not change over the course of an individual's lifetime. However, it can be recruited and brought into action by the adaptive immune system. The Classical pathway of activation of the complement system is a group of blood proteins that mediate the specific antibody response. [source: Wikipedia] The Classical pathway begins with circulating C1Q binding to an antigen on the surface of a pathogen, which goes on to active and recruit 2 copies of each C1R and C1S, forming a C1 complex. The activated C1 complex cleaves C2 and C4. Activated cleavage products C2A and C4B combine to form C3 convertase, which cleaves C3. The cleavage product C3B joins the complex to form C5 convertase, which cleaves C5. The cleavage product C5B joins C6, C7, C8 and multiple copies of C9 to form the Membrane Attack Complex, which forms a channel for water to flood into the target cell, leading to osmotic lysis. The Decay accelerating factor (DAF) inhibits C3 convertase. The Lectin pathway involves mannose-binding lectin (MBL) binding the surface of the pathogen instead of C1Q. MBL-associated serine proteases MASP1 and MASP1 can cleave C2 and C4 in place of the C1 complex, leading to the formation of C3 convertase and the subsequent cascade. The Alternative pathway relies on the spontaneous hydrolysis of C3 and the cleavage of factor B (CFB) by factor D (CFD), which form an alternative C3 convertase stabilized by factor P (CFP). Additional copies of the cleavage product C3B are recruited to the complex, resulting in an alternative C5 convertase, which cleaves C5 and contributes C5B to the formation of the Membrane Attack Complex.

补体系统是一种生物化学级联反应，它辅助或补充抗体清除病原体从生物体中的能力。它是称为先天免疫系统的免疫系统的组成部分，先天免疫系统不具有适应性，且在个体一生中不会发生变化。然而，它可以被适应性免疫系统动员并投入使用。补体系统的经典激活途径是一组介导特异性抗体反应的血液蛋白。[来源：维基百科] 经典途径始于循环中的C1Q与病原体表面的抗原结合，进而激活并募集两个C1R和C1S的副本，形成C1复合物。激活的C1复合物裂解C2和C4。激活的裂解产物C2A和C4B结合形成C3转化酶，进而裂解C3。裂解产物C3B加入复合物，形成C5转化酶，裂解C5。裂解产物C5B与C6、C7、C8以及多份C9结合，形成膜攻击复合物，该复合物形成水涌入目标细胞的通道，导致渗透性溶解。衰变加速因子（DAF）抑制C3转化酶。凝集素途径涉及甘露糖结合凝集素（MBL）与病原体表面的结合，而非C1Q。MBL相关丝氨酸蛋白酶MASP1和MASP2可替代C1复合物裂解C2和C4，导致C3转化酶的形成以及随后的级联反应。替代途径依赖于C3的自发水解以及因子D（CFD）对因子B（CFB）的裂解，从而形成由因子P（CFP）稳定的替代C3转化酶。额外的裂解产物C3B被募集到复合物中，结果形成替代C5转化酶，裂解C5，并贡献C5B以形成膜攻击复合物。