What does "left shift" refer to?

Consider an analogy. Your country has 1000 full-time soldiers (the standing army) who are ready at a moment's notice to defend you from any invaders. When the attack eventually comes, however, you soon realise that 1000 soldiers may not be enough, and so you also call up another 500 reserves. But even this number isn't sufficient. What can you do?

One solution, historically often employed in times of dire need, is to push into service inexperienced soldiers-in-training and even young men. Even though they may not be as efficacious, per person, as a fully trained soldier, they are still better than nothing.

The war, of course, is an analogy of what happens every time our bodies meet a foreign pathogen - a vicious immunological battle breaks out between ourselves and the invader. In this context, more white blood cells (predominantly neutrophils) are required, and the bone marrow starts hyper-producing them, pushing out the additional supply as soon as it is ready. However, a further option is to dispatch slightly immature forms of the neutrophils - neutrophils-in-training if you will - on to the frontline.

This is what "left shift" refers to (at least in the context of medicine, anyway). The immature neutrophil forms tend to have less lobularity to the nuclei; their nucleus tends to be shaped like a curved band, hence 'band cells'.

As far as I know, the odd name comes from the fact that textbooks traditionally showed the haematopoeitic* series from left (most immature) to right (mature cell) - and so a 'left shift' would equate to an increase in the proportion of immature cells in the peripheral blood.


* blood cell development