Here’s a great question from a reader.
Q. Why are Howell-Jolly bodies seen in sickle cell anemia? That’s where I am getting caught up. Also, what is the pathophysiology behind having nucleated RBCs?
A. The pathophysiology behind Howell-Jolly bodies and nucleated red blood cells in patients with sickle cell anemia is the same: the patient has effectively lost his or her spleen.
Pretend you’re a sickle cell for a minute. You’re stuck in this strange, elongated shape, and you’re not deformable like you once were. Floating through big vessels is easy, but once you get to capillaries, it becomes difficult to squeeze through without getting stuck. Once you get stuck, other red cells (both sickled and non-sickled) pile up behind you, and pretty soon, no blood is going through that vessel at all.
Now imagine the spleen, with its difficult-to-navigate circulation. It’s easy to see how you’d get frequent infarcts in the spleen. And every time an infarct heals, it leaves behind a little bit of fibrotic scar tissue. Eventually, over a period of years, the spleen becomes a useless little nubbin of tissue, and we say the patient has undergone “autosplenectomy” (which is kind of silly, because that makes it sound like the patient took a knife and did self-surgery, but whatever).
So whatever the spleen normally filters out of the blood – like nucleated red cells, unusually-shaped or damaged red cells, or little bits of iron or nuclear remnants in red cells – will now remain in the blood. Howell-Jolly bodies are little, unextruded nuclear remnants in red cells (sometimes a red cell just doesn’t get all of the nucleus out – but normally macrophages in the spleen will remove that last little bit). You can see one right under the upper nucleated red cell in the image above. The bits of iron are called Pappenheimer bodies (splenic macrophages remove those too. Target cells are also visible in peripheral blood because the spleen is no longer there to remove them.
The platelet count is up too, by the way, in patients without a spleen. This is because normally, the spleen houses about 30% of the platelets in the body. Remove the spleen, and you take away their home (that’s sad!), so they end up just circulating in the blood.
The constellation of these findings (nucleated red cells, Howell-Jolly bodies, Pappenheimer bodies, target cells, and an increased platelet count) is called a “post-splenectomy blood picture.” It’s seen in anyone without a spleen (not just in patients with sickle cell anemia).
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