FIGURE 2.

Complement system and COVID-19. The complement system is represented as well as its contribution to cell and tissue injury in COVID-19 and therapeutic approaches that have been tested in isolated patients or which are undergoing clinical trials. The classical, alternative and mannose-binding lectin pathway for complement activation as well as extrinsic factors that may contribute to complement activation are displayed. C3aR1 and C5aR1 are cell surface receptors activated by C3a and C5a, respectively. Transmembrane serine protease 2 (TMPRSS2) is a cell surface enzyme that processes SARS-CoV-2 protein S, facilitating binding to ACE2, which functions as a cell surface receptor for SARS-CoV-2, allowing virus entry into cells. Purple proteins represent extrinsic pathways for complement activation, dark blue continuous lines represent protein processing or protein incorporation into protein complexes and green lines represent therapeutic agents inhibiting targets. Thin discontinuous lines represent enzymatic activity.