Haemosiderin is an insoluble iron-storage protein — a golden-brown pigment formed when phagocyte cells break down haematin from old haemoglobin. It represents the body’s overflow iron store: when iron supplies exceed the capacity of ferritin, the soluble storage protein, the excess is packed away as haemosiderin granules in the liver, spleen, and bone marrow. Normally a modest reserve, it accumulates massively in iron-overload disorders such as haemochromatosis and in tissues after repeated bleeding, where its rusty deposits discolour skin and organs. Pathologists recognise it by its characteristic staining with Prussian blue. Though poorly mobilised compared with ferritin iron, it testifies to the body’s determination never to waste this precious metal.