Illustration of a human heart affected by amyloid light chain (AL) amyloidosis. Amyloidosis is a rare disease in which abnormal proteins known as amyloid fibrils (white) accumulate on heart muscle tissue. Amyloid light chain (AL) amyloidosis is the most common form of systemic amyloidosis and is caused by the misfolding of immunoglobulin light chains, which are produced by abnormal plasma cells in the bone marrow. Over time, these light-chains aggregate to form fibrils, which build up around and in between the cardiomyocytes (heart muscle cells). This stiffens the muscle tissue, impairing the hearts ability to relax and fill with blood. Deposits can be widespread, affecting multiple organs, or they can be more localised and symptoms vary accordingly.