Sickle cells: erythrocytes deformed by hemoglobin S

In a normal blood smear under the light microscope
(1), erythrocytes (red blood cells) appear
as regular round disks of about 7 μm diameter.
Since a normal red blood cell is nonnucleated
and biconcave, the center appears paler than
the periphery. The erythrocytes of affected persons
are deformed and resemble sickles (2).
However, even the non-sickle-shaped red cells
display unusual sizes and shapes. In the course
of the disease, acute crises called sickle crises
(3) occur, during which sickle-like cells are
greatly increased and completely dominate the
blood picture. Heterozygotes show occasional
sickle cells but do not suffer from sickle crises,
and at the most have only very mild signs and
symptoms.