From OMIM
Sickle cell disease is a multisystem disease associated with episodes of acute illness and progressive organ damage. Hemoglobin polymerization, leading to erythrocyte rigidity and vasoocclusion, is central to the pathophysiology of the disease, but the importance of chronic anemia, hemolysis, and vasculopathy has been established. The most common cause of sickle cell anemia is the HbS variant (141900.0243), with hemoglobin SS disease being most prevalent in Africans (review by Rees et al., 2010). See review of infection in sickle cell disease by Booth et al. (2010). Piel et al. (2017) reviewed the genetic and nongenetic modifiers of the severity of sickle cell disease.  http://www.omim.org/entry/603903

From MedlinePlus Genetics
Sickle cell disease is a group of disorders that affects hemoglobin, the molecule in red blood cells that delivers oxygen to cells throughout the body. People with this disease have atypical hemoglobin molecules called hemoglobin S, which can distort red blood cells into a sickle or crescent shape.

Signs and symptoms of sickle cell disease usually begin in early childhood. Characteristic features of this disorder include a low number of red blood cells (anemia), repeated infections, and periodic episodes of pain. The severity of signs and symptoms varies from person to person. Some people have mild health issues, while others are frequently hospitalized for more serious complications. 

The signs and symptoms of sickle cell disease are caused by the sickling of red blood cells. When red blood cells sickle, they break down prematurely, which can lead to anemia. Anemia can cause shortness of breath, fatigue, and delayed growth and development in children. The rapid breakdown of red blood cells may also cause yellowing of the skin and whites of the eyes (jaundice). Painful episodes can occur when sickled red blood cells, which are stiff and inflexible, get stuck in small blood vessels. These episodes deprive tissues and organs, such as the lungs, kidneys, spleen, and brain, of oxygen-rich blood and can lead to organ damage. A particularly serious complication of sickle cell disease is high blood pressure in the blood vessels that supply the lungs (pulmonary hypertension), which can lead to heart failure. Pulmonary hypertension occurs in about 10 percent of adults with sickle cell disease.

There are currently a range of treatment options for people with sickle cell disease. Some treatments address the symptoms of the condition, while others address the genetic cause of sickle cell disease and effectively cure the condition.  Without treatment, individuals with sickle cell disease often have lifelong health problems.  https://medlineplus.gov/genetics/condition/sickle-cell-disease