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| |    The Immune System in Sickle Cell Disease by Lynn Cates, M.D., F.A.A.P. reviewed by Lewis Hsu, M.D., Ph.D. The body's immune system can fight infections in many ways, but some of these defenses don't work normally in children with sickle cell disease. A child with this disease usually can handle common respiratory viral infections like colds with careful supportive care, such as fever control and liberal use of fluids, but you never know which fever actually is a more serious bacterial infection. Also, these infections may make him more susceptible to serious secondary infections caused by bacteria. For this reason, healthcare providers have learned to promptly see sickle cell children for any fever, and to treat almost all cases of fever with antibiotics. Even seemingly mild infections that other family members manage easily can set the stage for serious health problems in a child with sickle cell disease. Immune factors Some of the most severe infections these children develop are caused by bacteria that have an outer coating, or capsule, protecting them from the body's immune system. These are called encapsulated bacteria, an excellent example of which is the pneumococcus. One important way that children's bodies fight off encapsulated bacteria like pneumococci is by coating them with immune factors in the bloodstream so that the bacteria can be recognized by the body's immune cells and consequently killed. This process, called opsonization, is similar to coating bread (the bacterium) with butter and jelly (immune factors) to make it more delectable. Opsonized bacteria are much more likely to be recognized by immune cells in the bloodstream, eaten by them, and killed. But because children with sickle cell disease have decreased ability to opsonize, these bacteria stand a better chance of evading the body's immune system and causing infection. The spleen The spleen is a large organ located in the upper left area of the abdomen, tucked up under the lower part of the rib cage. It consists of a vast network of tiny blood vessels and immune cells including lymphatics, which help to make antibodies, and macrophages, which eat and kill germs. It's been known for many years the important role the spleen plays in protecting against life-threatening infections, particularly those with encapsulated organisms like the pneumococcus. This organ protects children from serious infection by producing immune factors and by filtering bacteria out of the bloodstream when they try to navigate its huge maze of tiny blood vessels. In essence, immune cells in the spleen pluck bacteria out of the circulation and kill them. However, in most children with sickle cell, beginning in infancy, the spleen starts to become damaged by sickled red blood cells. The abnormal shape of sickled cells causes them to stick in tiny blood vessels of the spleen, cutting off circulation (infarct) to the area supplied by those vessels and causing that region to die. The spleens of most children with sickle cell disease have been damaged so badly in the first couple of years of life, it's as if their spleen had been surgically removed. When the spleen no longer helps at all in protecting against infections, the child is said to have "functional asplenia" (or the same lack of spleen function as would be seen in someone with no spleen at all). In other children with sickle cell disease, the spleen repeatedly traps red blood cells and becomes enlarged--a process that serves as one indication for surgical removal and results in true asplenia. In summary, children with sickle cell disease have a harder time fighting infections because of the decreased function of immune factors in the blood and decreased splenic function. Therefore, they are at a markedly higher risk of developing serious, potentially fatal, infections than other children. It is extremely important for the family to let the healthcare providers know that this is a child with sickle cell disease when discussing how to handle a fever. Advice given for fever in most children is not the same as advice for fever in a child with sickle cell.
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