A left shift is an increase in the number of immmature neutrophils or polymorphonuclear leukocytes (PMN's). These are called band or stab neutrophils, and in more severe cases there may be metamyelocytes or myelocytes present. Mature (normal) neutrophils are referred to as segmented neutrophils or segs. In a normal differential there should be 0-6 bands per 100 WBC's and no metamyelocytes or myelocytes.
These immature cells make it into the bloodstream because chemical signals (cytokines, complement, etc.) released from the site of inflammation stimulate the bone marrow to release them prematurely. They continue to mature after entering the bloodstream, and the left shift will usually disappear within a few days if the cause of the inflammation is addressed.
An increased neutrophil count with a left shift is typically associated with bacterial infection, and may be increased in other inflammatory conditions or post-surgically.