Red blood cells present in the blood are normally detached from each other. Rouleaux are clumps of red blood cells that look like stacked plates. They usually form as a result of abnormal quantities of certain proteins (immunoglobulin, fibrinogen) in the blood. Rouleaux are a non-specific indication of the presence of a pathology.
Red blood cell rouleaux are found in chronic inflammatory states and infections, multiple myeloma (excess synthesis of a single class of immunoglobulin by the bone marrow) and sometimes during pregnancy due to increased fibrinogen, the main coagulation protein.