Illustration of a blood smear of a patient with severe aplastic anaemia (SAA). SAA is the underproduction of all blood cell types normally produced in the bone marrow by stem cells. This blood smear shows a significant decrease in the number of red blood cells (red) and platelets (purple), and a complete absence of leukocytes. In many cases of SAA, the cause of the disease is unknown, however research suggests that autoimmune responses might be involved.