A complete blood count includes tests of red blood cells (RBCs), platelets, and white blood cells (WBCs). The white blood cell count tells the total number of white cells in a sample of blood, and the differential tells the percentage of each type of white blood cell. By convention (since before computer printouts), laboratories reported the WBC and differential in the same order (from left to right): WBC total, bands (immature neutrophils), neutrophils, eosinophils, basophils, lymphocytes, and monocytes. This is the basis for a "shift to the left."
The total percentages of the differential (the percentage amount of each type of cell) must add up to 100%. You can calculate the actual number of each type of white blood cell based on its percentage. So, if your total white blood cell count is 8000, the bands are 3%, and the neutrophils are 50%, the total number of bands is 240 (3% of 8000) and the total number of neutrophils is 4000 (50% of 8000).
The first 3 numbers from the left (WBC total, bands, and neutrophils) are important because the total number of white blood cells increases when you have an acute infection, and the numbers of bands and neutrophils also increase, causing a shift in percentages because, as the percentage of bands and neutrophils increase, the percentages of the other cells must decrease. This then constitutes the "shift to the left."
White Blood Cells
White blood cells vary in number because they are part of the immune system of the body. Some remain dormant in the spleen and other lymph tissue until they are activated as part of the defense system of the body, increasing the number. Additionally, infection stimulates the bone marrow to produce more white blood cells. While normal values range from 5000 to 10,000 in most people, an acute infection (such as appendicitis) can increase this number to 15,000 or higher. (A very high count may also indicate leukemia.)
Bands and Neutrophils
Bands are immature neutrophils, also sometimes called "stabs," which is German for "rods." According to Davis's Comprehensive Handbook of Laboratory and Diagnostic Tests, most neutrophils (also called "segs" or "polys") are mature (comprising 50 to 62% of the total white blood cell count) and the percentage of bands is low (3 to 6%). Mature neutrophils are polymorphonuclear, so the nucleus of the cell is divided (poly) and segmented. The bands, on the other hand, have not matured, so they have a band/rod shape.
When infection occurs, the bone marrow starts to rapidly produce neutrophils because they are your body's main defense against bacterial infection. As large numbers of neutrophils are released into the bloodstream, increasing numbers are immature bands; so an increase in bands is the primary indicator of a shift to the left. For example, with an acute infection, the white blood cell count may increase to 15,000, the bands to 10% and the neutrophils to 65%. (The other cells will total 25%).