this is a lifelong disease which starts in childhood. the red blood cells take up an abnormal, rigid, sickle shape. the cells also become sticky. this causes difficult blood flow when cells flow through long narrow capillaries. low oxygen increases the problem. as they pass through low oxygen areas most cells take up this shape. the cells then stick to the inner wall of blood vessels, especially the branching point of veins. this leads to a blockade of blood flow in many organs. severe complications may result.