Complement C5a Receptor 1 (C5aR1 or CD88), a G-protein coupled receptor (GPCR) that is highly expressed on myeloid cells such as neutrophils, monocytes, macrophages, and mast cells. Its ligand, C5a, is a powerful anaphylatoxin generated upon activation of the complement cascade, a key component of the innate immune system. The C5a-C5aR1 axis is a major driver of acute and chronic inflammation. Upon binding, it triggers a robust pro-inflammatory response: promoting chemotaxis (directed migration) of neutrophils to sites of inflammation or infection, enhancing oxidative burst, increasing vascular permeability, and inducing the release of additional cytokines. In pathological contexts like severe COVID-19 or cancer, excessive or dysregulated C5a production leads to tissue damage and immune dysregulation.