Researchers believe this is caused by an autoimmune reaction, a condition in which the natural defense system of the body begins attacking itself. In SAA the immune system begins attacking the bone marrow. Red blood cells are responsible for carrying oxygen to all of the organ systems in the body, and low numbers (anemia) can cause difficulty breathing and fatigue. Platelets are responsible for normal blood clotting and low numbers can result in easy bruising and bleeding which can be deadly. White blood cells are responsible for fighting infections, and low numbers of these can lead to frequent infections, the most common cause of death in patients with aplastic anemia.