Hemosiderin is an iron-based pigment derived from hemoglobin.

Free iron is quite toxic and thus iron is immediately bound to an enzyme, transferrin, for transport throughout the body. When transferrin reached a cell requiring iron, the iron is complexed onto a protein, apoferritin, for sequestration within the cell. Several apoferritin complexes combine to form a micelle which is called ferritin.

It follows that if a large amount of iron is made available to a cell that it will produce a large number of ferritin micelles. Such large aggregates of micelles are referred to as hemosiderin and they are visible as inclusions within cells when viewed under a light microscope.

Conditions leading to large amounts of iron being avaible either systemically or locally include hemolysis, severe congestion, dramatic increases in dietary iron consumption, or increased absorption or decreased loss of iron by the body.

The presence of hemosiderin in small amounts within iron rich tissues such as the spleen, liver, and bone marrow is considered normal. Large aggregations of hemosiderin or its presence in tissues such as the lung or subcutaneous connective tissue suggest a pathological condition. 

The most obvious and perhaps common cause of hemosiderin deposition is simple bruising. Trauma to the skin causes rupture of vessels and subsequent hemorrhage. The area of hemorrhage is first visible on the skin as a typical purplish color &endash; this is the color of the red blood cells. As the erythrocytes are lysed and scavenged away, the wound turns a yellowish brown &endash; the color of the hemosiderin remaining in the tissues. Eventually the bruise fades as the hemosiderin is removed by macrophages.