Red Blood Cells (erythrocytes) are packed with hemoglobin, an oxygen binding protein. Normal erythrocytes are ovoid and flexible, and pass easily through the small diameters of the capillaries. However, in people with Sickle Cell Anemia, the erythrocytes can become sickled at low oxygen concentrations. Sickled erythrocytes easily become tangled and create blockages in the capillaries, cutting off blood supply and oxygen to the tissues. The eventual result is severe organ damage and premature death.
 

PLEIOTROPIC EFFECTS

During a sickling crisis many tissues and organs are damaged because their blood supply is cut off.   Moreover red blood cells cannot regain their normal shape once they have become sickled.  They are therefore removed from the blood supply by the spleen.  This results in a severe and debilitating under supply of erythrocytes - a condition known as anemia. 

This leads to many widespread and different types of damage, called pleiotropic effects, all of which however share a single primary cause - polymerization of HbS.
 

  See an enlarged view of this diagram showing the pleiotropic effects.
.Strickberger, MW.  Genetics, 3rd ed., Macmillan, 1985.  p 543