The abnormal proliferation of reticulum cell (histiocytes) which infiltrate various organs and. Macrophages destroy blood cells causing blood abnormalities. Meningoencephalitis frequently occurs when the histiocytes infiltrate the mininges and cerebral tissue. Symptoms start at birth or soon after and become progressively worse without treatment. Medication can control the condition but a hematopoietic stem cell transplant is needed to achieve remission. See also Hemophagocytic reticulosis