Hemoglobin S decreases the amount of oxygen in the blood cells.  These red blood cells become deformed, stiff, and rigid, changing their shape from disc-shaped with a slight indentation in the middle, to sickle-cell shaped.  The distorted figures are unable to distribute the normal amount of oxygen to the body and can sometimes clog the blood vessels, resulting in painful blood clots.

A patient almost always has at least one type of crisis in his/her life.  However, how often this occurs depends on the severity of the disease.  This can vary from once every few years to a great many in one year.   The most common of the crises is one in which the sickle-shaped cells stack on top of one another and block a capillary.  This can result in tissue damage or even death.  Some of these types of crises are:

Aplastic Crisis
Splenic Sequestration Crisis
Hemolytic Crisis

Aplastic Crisis occurs when infected bone marrow becomes unable to produce red blood cells. Splenic Sequestration Crisis occurs when the spleen becomes too large and blocks the flow of blood. Hemolytic Crisis occurs when red blood cells unexpectedly break down.