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."
Why do you see Howell-Jolly bodies in sickle cell anemia?