Ferritin is present in every cell type. It serves to store iron in a non-toxic form, to deposit it in a safe form, and to transport it to areas where it is required.  The function and structure of the expressed ferritin protein varies in different cell types.  This is controlled primarily by the amount and stability of messenger RNA (mRNA), but also by changes in how the mRNA is stored and how efficiently it is transcribed.  One major trigger for the production of many ferritins is the mere presence of iron; an exception is the yolk ferritin of Lymnaea sp., which lacks an iron-responsive unit.
Free iron is toxic to cells as it acts as a catalyst in the formation of free radicals from reactive oxygen species via the Fenton reaction. Hence vertebrates have an elaborate set of protective mechanisms to bind iron in various tissue compartments. Within cells, iron is stored in a protein complex as ferritin or the related complex hemosiderin. Apoferritin binds to free ferrous iron and stores it in the ferric state. As ferritin accumulates within cells of the reticuloendothelial system, protein aggregates are formed as hemosiderin. Iron in ferritin or hemosiderin can be extracted for release by the RE cells, although hemosiderin is less readily available. Under steady-state conditions, the level of ferritin in the blood serum correlates with total body stores of iron; thus, the serum ferritin FR5Rl is the most convenient laboratory test to estimate iron stores.
Because iron is an important mineral in mineralization, ferritin is employed in the shells of organisms such as molluscs to control the concentration and distribution of iron, thus sculpting shell morphology and colouration.  It also plays a role in the haemolymph of the polyplacophora, where it serves to rapidly transport iron to the mineralizing radula.
Iron is released from ferritin for use by ferritin degradation, which is performed mainly by lysosomes.