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. The complex has several synonyms, including MRP8-MRP14, calgranulin A and B, cystic fibrosis antigen, 60BB antigen, and 27E10 antigen.
Calprotectin becomes available in the intestinal lumen via leukocyte shedding, active secretion, cell disturbance, and cell death. This results in elevated faecal calprotectin levels, which can be detected in the stool. Elevated faecal calprotectin levels therefore indicate migration of neutrophils into the intestinal mucosa, which occurs during intestinal inflammation. As people with active inflammatory bowel diseases (IBD) such as ulcerative colitis or Crohn disease have as much as a 10-fold increase in faecal calprotectin levels, the measurement of faecal calprotectin can serve as a biochemical test for these diseases.