Here’s a long term: leukoerythroblastotic reaction. Despite its length, it’s a pretty good term, because it describes a reactive condition in which you see young red cells (erythroblasts) and young white cells (leuko-) out in the peripheral blood. When you have a bunch of immature neutrophils (like the myelocyte, above right) in the blood, it’s called a left shift. A left shift usually indicates that the patient has an infection somewhere. If you can find toxic changes in the neutrophils (toxic granulation, Dohle bodies, or cytoplasmic vacuolization), that would clinch the infection diagnosis.

Sometimes, however,  in addition to a left shift, you’ll see early red cell precursors (see above left) – the so-called leukoerythroblastotic reaction (LEBR). When you see this, you need to consider things besides infection. Sometimes, a LEBR can be a physiologic thing – but sometimes, it indicates a serious underlying problem.

If the hemoglobin is very low (for whatever reason – severe iron deficiency, massive blood loss), the bone marrow will try very hard to make new red cells and send them out into the blood as fast as possible. Sometimes, it is a little overzealous, and it lets a few red cell precursors (nucleated red cells) slip out of the marrow too. And sometimes, it is so freaked out that it starts letting neutrophil precursors (metamyelocytes, myelocytes, promyelocytes) out too! This is a normal response to a severe anemia. A LEBR in this setting is physiologic (it’s just a part of the bone marrow’s appropriate response).

Sometimes, however, a LEBR is pathologic. If the marrow is full of something besides hematopoietic tissue – say, for example, a carcinoma, or a leukemia – then the hematopoietic cells will not have enough room and space to mature properly. They will end up leaving home before they are ready, and you’ll see both nucleated red cells and neutrophil precursors in the blood. This is an ominous sign.

One little trick you can use to determine whether a LEBR is worrisome is to look at the hemoglobin. If the hemoglobin is very low (< 6 g/dL), then the  LEBR is probably physiologic. However, if the hemoglobin is normal, or only slightly decreased, then there is no good reason for the patient to have a  leukoerythroblastotic reaction, and you’d better figure out what’s causing it.