Sickle cell anemia is one of a larger group of inherited red blood cell disorders called hemoglobinopathies. Hemoglobinopathies are disorders in which one of the hemoglobin chains (usually the beta chain) is abnormal. There are a ton of different hemoglobinopathies. Some cause no clinical symptoms, but others (like sickle cell anemia) can be devastating.

In sickle cell anemia, the hemoglobin abnormality consists of a point mutation in the beta chain gene for hemoglobin; the resulting abnormal gene product is denoted HbS. If you are heterozygous for the HbS gene (meaning you have inherited only one abnormal beta chain gene), you will have what is called sickle trait, which is asymptomatic (because your remaining normal beta chain gene is able to produce sufficient quantities of normal beta chains). If you are homozygous for the HbS gene (meaning you have inherited two abnormal beta chain genes), you will get sickle cell disease, which is symptomatic in most patients.

The problem with HbS is that as it releases oxygen, it polymerizes and aggregates with other HbS molecules, making the red cell stiff and distorted. These distorted, sickle-shaped red cells are fragile (they have a hard time negotiating the splenic sinusoids because they aren’t nice and pliable like normal red cells), so the patient can end up with a hemolytic anemia. They also can clump together, especially in small vessels, so the patient may end up with ischemic tissue damage.

There are a number of findings in the blood smear of a patient with sickle cell disease, the most prominent of which are sickle cells (as you can see at the center of the above image). These are usually crescent-shaped, but they can be sort of needle-shaped or holly-leaf-shaped too.

In patients with long-standing disease, you may also see what is termed a “post-splenectomy blood picture.” Over time, as the sickle cells try to make their way through the spleen but end up getting stuck, the spleen becomes infarcted and fibrotic, eventually winding up as a small, useless lump of fibrous tissue (basically, the patient has no viable spleen left at all). In patients with no spleen, whether it is due to chronic sickle cell disease or surgical removal, there some characteristic findings in the blood. These include nucleated red cells (normally, these are present only in the bone marrow; if they make it out of the bone marrow, the spleen eats them up), target-shaped red cells, Howell-Jolly bodies (little, unextruded nuclear remnants that the spleen normally removes), and Pappenheimer bodies (little clusters of iron molecules that the spleen also normally removes). The platelet count is usually elevated too. You can see all of these findings in the image above. The large, dark purple, single inclusions are Howell-Jolly bodies, and the smaller, greenish-gray granules are Pappenheimer bodies.