What is comprised in the alternative C5 convertase complex?

The alternative C5 convertase complex is specifically made up of two molecules of C3b and one molecule of Bb, forming the structure referred to as C3b2Bb. This complex plays a critical role in the complement system, particularly in the activation process that leads to inflammation and pathogen lysis.

Once the alternative pathway is activated, C3b binds to a target surface and is then stabilized by the presence of factor B, which is cleaved by factor D to form Bb. The C3b2Bb complex functions effectively to cleave C5 into C5a and C5b, which are crucial for initiating the formation of the membrane attack complex and for attracting inflammatory cells to the site of infection.

Understanding the composition of the alternative C5 convertase complex is vital because it exemplifies how the immune system amplifies responses to pathogens, contributing to innate immunity. In contrast, C3bBb is associated with the C3 convertase rather than the C5 convertase, while C4b2a pertains to the classical pathway and C3aC5a represents cleaved products rather than a functional convertase.