Ferritin is a universal intracellular and extracellular protein that stores iron and releases it in a controlled fashion.

The protein is produced by almost all living organisms, including archaea, bacteria, algae, higher plants, and animals.

It is the primary intracellular iron-storage protein in both prokaryotes and eukaryotes, keeping iron in a soluble and non-toxic form.

[3] Ferritin is found in most tissues as a cytosolic protein, but small amounts are secreted into the serum where it functions as an iron carrier.

[4] Aggregated ferritin transforms into a water insoluble, crystalline and amorphous form of storage iron called hemosiderin.

[8] In human ferritin, introns are present between amino acid residues 14 and 15, 34 and 35, and 82 and 83; in addition, there are one to two hundred untranslated bases at either end of the combined exons.

[11] The nature of these subunits varies by class of organism: All the aforementioned ferritins are similar, in terms of their primary sequence, with the vertebrate H-type.

[12] Some ferritin complexes in vertebrates are hetero-oligomers of two highly related gene products with slightly different physiological properties.

Cytosolic ferritin shell (apoferritin) is a heteropolymer of 24 subunits of heavy (H) and light (L) peptides that form a hollow spherical nanocage that covers an iron core composed of crystallites together with phosphate and hydroxide ions.

The mitochondrial ferritin's Ramachandran plot[15] shows its structure to be mainly alpha helical with a low prevalence of beta sheets.

By contrast, organisms such as Pseudomonas, although possessing endotoxin, cause plasma ferritin levels to drop significantly within the first 48 hours of infection.

[26] The concentration of ferritin has been shown to increase in response to stresses such as anoxia,[27] which implies that it is an acute phase protein.

[26] Serum ferritin levels are measured in medical laboratories as part of the iron studies workup for iron-deficiency anemia.

Although vegetarian adults have lower iron stores than non-vegetarians, their serum ferritin levels are usually within the normal range.

Ferritin and C-reactive protein may be possible screening tools for early diagnosis of systemic inflammatory response syndrome in cases of COVID-19.

[43][44] According to a study of anorexia nervosa patients, ferritin can be elevated during periods of acute malnourishment, perhaps due to iron going into storage as intravascular volume and thus the number of red blood cells falls.

The measurement of ferritin through immunoassay or immunoturbidimeteric methods may also be picking up these isoferritins thus not a true reflection of iron storage status.

This explains why ferritin levels remain relative low in hereditary hemochromatosis, while transferrin saturation is high.

As a consequence, elevated hepatic and serum ferritin levels are consistently reported in chronic liver diseases.

[51][52][53] Studies showed association between high serum ferritin levels and increased risk of short-term mortality in cirrhotic patients with acute decompensation[54] and acute-on-chronic liver failure.

[55] An other study found association between high serum ferritin levels and increased risk of long-term mortality in compensated and stable decompensated cirrhotic patients.

[57][7]: e9  Sam Granick and Leonor Michaelis produced apoferritin in 1942[7]: e10 Ferritin is used in materials science as a precursor in making iron nanoparticles (NP) for carbon nanotube growth by chemical vapor deposition.

[60] Cavities formed by ferritin and mini-ferritins (Dps) proteins have been successfully used as the reaction chamber for the fabrication of metal nanoparticles.

[6][65] Experimental COVID-19 vaccines have been produced that display the spike protein's receptor binding domain on the surface of ferritin nanoparticles.