Light micrograph of amyloidosis in a human spleen. Amyloidosis is caused by abnormal folding of normal soluble proteins leading to fibril formation. These clumps of protein are called amyloid deposits. The spleen is involved in most cases of systemic amyloidosis. The micrograph shows a 'lardaceous spleen', in which amyloid is deposited in the walls of the splenic sinusoids.