Light micrograph of macrophages (a type of blood cell, the round bubbly or brown individual circles) mixed with red blood cells (small darker red circles) and colloid, the light pink substance in the background. Colloid is the material found in the thyroid which helps produce thyroid hormones. Note that the brown colour in many macrophages is due to the presence of hemosiderin pigment, a product of breakdown of blood. The presence of macrophages and blood in this thyroid is due to cystic change and haemorrhage (bleeding). This change is benign and is frequently seen by pathologists trying to exclude a diagnosis of cancer by examining thyroid tissue samples. Haematoxylin and eosin stained tissue section. Magnification: 200x when printed at 10cm.