Calprotectin is a complex of the mammalian proteins S100A8 and S100A9. In the presence of calcium, calprotectin is capable of sequestering the essential nutrients manganese and zinc. This metal sequestration affords the complex antimicrobial properties. Calprotectin is the only known antimicrobial manganese sequestration protein complex. Calprotectin comprises as much as 60% of the soluble protein content of neutrophil cytosol, and is secreted by an unknown mechanism during inflammation. Faecal calprotectin has been used to detect intestinal inflammation, and can serve as a marker for inflammatory bowel diseases.

Calprotectin has a high affinity for calcium, zinc and manganese. Calprotectin is the only known antimicrobial agent that acts through manganese sequestration.