Ferritin is a protein that can take up, store and release iron molecules in a controlled manner. It is found in almost all living organisms. In humans, it regulates the amount of iron in the body, preventing both iron toxicity and iron deficiency. Almost all cells can produce ferritin and will produce it in response to iron. Cells also produce ferritin in response to infection and cancer, apparently in an attempt to deny iron to foreign cells. Smaller amounts appear in blood plasma where it plays a role transporting iron to different parts of the body. When bound to ferritin, iron is both water-soluble and non-toxic where in its raw form, it is both insoluble and toxic.
The level of ferritin in a blood sample is roughly dependent on the amount of iron in the body and, as such, doctors routinely test for it to determine if iron levels are abnormally low or high.