Within the red
blood cell is a
substance called hemoglobin, whose
function is to carry oxygen. Hemoglobin has a
certain chemical structure known as hemoglobin A. In
about 0.3 percent of U.S. African Americans, there is an inherited disease in which an abnormality
occurs in the structure of the hemoglobin. This
abnormally configured hemoglobin is
called hemoglobin S, or
sickle hemoglobin.
Two
forms of inheritance occur. The abnormal hemoglobin may be present in only one of the parents and the child can only inherit a single gene and its cells will contain only a
small proportion of the
abnormal pigment, In such
cases little harm will result. If both parents
carry the gene, however, the child may be homozygous, causing the red
blood cells to contain a large amount of the abnormal pigment. Serious
illness may then follow.
The presence of
large amounts of hemoglobin S in the red
blood cells results in a tendency for the red blood cells to
become distorted in a
crescent shape ("sickle cells") if the level of
oxygen in the arterial
blood falls. This sickling can be identified by
microscopic examination of the blood.
Sickle-cell disease
refers to the inheritance of hemoglobin S from both parents with large
amounts of sickle hemoglobin in the cells. Even
slight degrees of oxygen lack will result in a marked "sickling" change in the blood and
consequent obstruction to the
circulation in various parts of the body by
masses of sickled cells ("sickle-cell crisis"). During a sickle-cell crisis (and, to a lesser extent, at
other times as well), the sickle cells are
destroyed and liberate hemoglobin into the blood plasma in great quantity---sometimes enough to produce jaundice and to lower the red cell count seriously. Severe pain, dangerous organ damage, and even death may result.
Inheritance of a single dose of
hemoglobin S from one
parent results in a
condition called "sickle-cell trait."
Although abnormalities can be
noted on careful examination of the blood, usually sickle-cell trait rarely results in symptoms.