Sickle cell disease is an inherited disorder of hemoglobin due to which the red blood cell takes up a stiff sickle shape when exposed to stress like hypoxia, hypothermia and dehydration. This causes blood vessels to become occluded, and the patient will have symptoms depending on which vessel is impacted.
The severity of manifestation is inversely proportional to the levels of fetal hemoglobin in any given patient. Medications like Hydroxyurea increase levels of fetal hemoglobin resulting in alleviation of symptoms.