S100A8 (also known as MRP8, Calgranulin A, and CP-10) and S100A9 (also known as MRP14 and Calgranulin B) are secreted pro-inflammatory proteins that are upregulated in neutrophils and monocytes at sites of inflammation (e.g. psoriasis, rheumatoid arthritis, cardiac ischemia) and are present at elevated concentrations in rheumatoid arthritis synovial fluid. In the presence of calcium or zinc, S100A8 and S100A9 associate into noncovalent homodimers and heterodimers with each other. The heterodimer additionally binds and sequesters manganese, thereby restricting the growth of Mn-dependent bacteria. The S100A8/A9 heterodimer exhibits functions beyond those performed by the individual proteins. These include binding to fatty acids such as arachidonic acid and promoting astrocyte proliferation. S100A8, S100A9, and the heterodimer each promote neutrophil infiltration into sites of inflammation and inflammatory cytokine production by monocytes.