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Hematin is a brown-black pigment derived from hemoglobin. Although its precise composition has not yet been determined, it is known to contain heme iron in the ferric form. It is formed by the oxidation of heme from the ferrous Fe(II) to the ferric Fe(III) state. The pigment is associated most commonly with a severe hemolytic crisis after transfusion of incompatible blood or chronic parasitization of red blood cells by malarial protozoa. It is usually found in the cytoplasm of reticuloendothelial cells.
Acid hematin is a birefringent pigment, different from hemosiderin (refractile but not birefringent). Formalin pigment and malarial pigment are acid hematin which is not stained by the Prussian blue reaction, unlike hemosiderin which stains positively.
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