Sickle-cell isease or sickle-cell anaemia (or anemia) is a blood disorder characterized by red blood cells that assume an abnormal, rigid, sickle shape. Sickling decreases the cells' flexibility and results in their restricted movement through blood vessels, depriving downstream tissues of oxygen. The disease is chronic and lifelong: individuals are most often well, but their lives are punctuated by periodic painful attacks and a risk of various other complications. Life expectancy is shortened, with older studies reporting an average life expectancy of 42 and 48 years for males and females, respectively.