How could a breakdown of the immune system have led to anna's untimely death?

when the immune system is compromise, the body cannot strongly protect itself against bacteria or other pathogen causing disease and this is what happen to the sudden death of anna. her sickle cell disease deprived her of oxygen rich blood and her white blood cells that act as the body’s first line of protection did not function well. 

moreover, when foreign invaders attacked her body, she can no longer mounts a defense to fight off such bacteria that result to urinary tract infection that made her weaker. lastly, because she has type i diabetes the hormones in her body could not effectively perform feedback when she could have extremely low or high blood sugar, which could ultimately cause her to become unconscious or lifeless.

where is the scientific experiment?