Fig. 2.
Biological functions of the Complement System. Antibody production: CR2 expressed in B lymphocytes and follicular dendritic cells interacts mainly with C3d fragments covalently bound to the pathogen surface, activating B lymphocytes, and increasing antibody production. Opsonization: C3b and iC3b are responsible for covalently attaching to the surface of different pathogens and they interact with Complement receptors CR1, CR3 and CR4 present on the surface of phagocytic cells such as neutrophils and macrophages, facilitating phagocytosis. Inflammation: the anaphylatoxins C3a and C5a are powerful chemoattractants to inflammatory cells and induce the release of mediators that amplify the inflammatory response. They perform different functions such as stimulation of mast cells and basophils, causing their degranulation, and release of inflammatory mediators, increased vasodilation, activation of complex inflammasomes and cytokine secretion. Cell activation: the anaphylatoxins are responsible for recruitment and activation of inflammatory cells, including eosinophils, neutrophils, basophils, monocytes and B lymphocytes. Cell Lysis: the formation of the Membrane Attack Complex (C5b-9n) causes cell lysis. Immune Complexes Clearance: immune complexes covalently attached to C3b or C4b fragments binds to CR1 present on the surface of blood cells and are removed from circulation. This figure was created using Servier Medical Art, licensed under a Creative Commons Attribution 3.0 Unported License: https://smart.servier.com. Adaptations from the original art were made on the cell membrane and macrophages.