Neutropenia means an abnormally low number of neutrophils in the blood. Neutrophils (also known as polymorphonuclear leukocytes, granulocytes, segs, polys or PMNs) are a type of white blood cell (leukocyte) that is responsible for much of the body's protection against infection. They are readily attracted to foreign invaders and destroy them by engulfing or eating them.

Neutrophils release an enzyme that dissolves cells in the immediate area, resulting in the formation of pus. An inadequate number of neutrophils leaves the body at high risk for infection.

In babies, the most common cause is infection. Neutrophils are produced in the bone marrow and released into the bloodstream to travel to wherever they are needed. When neutrophils are being destroyed fighting infections, their numbers can become low in the blood because the bone marrow is not able to replace them as fast as they are being used.

There may be large numbers of immature forms of neutrophils, called bands, being produced by the bone marrow when the demand is high. Occasionally, a non-sick infant will have a low neutrophil count for no apparent reason.

A small sample of the baby's blood will be sent to the laboratory for a complete blood count (CBC). A CBC provides much information, such as how many and what types of red cells and white cells are in the blood. The white cells are counted, then they are looked at under a microscope to see what type of white cell they are (differential cell count).

There will be 100 white cells counted. If there are 50 neutrophils, then the neutrophil count is 50% of the total white cells. The other 50% will be a mixture of bands, lymphocytes, monocytes, eosinophils, and basophils which are other types of white cells.

Of primary importance is to investigate the source or the site of the infection and treat the underlying infectious process that caused the neutropenia. In rare cases when the neutrophil count is low enough to be life-threatening, the doctor may consider white cell transfusions from donor blood, medication to stimulate white cell production (Neupogen), or concentrated antibodies from pooled donors (intravenous immune globulin).

The outcome of the baby depends on the underlying cause of the neutropenia. Infection in newborns can certainly be life-threatening. A simple blood infection or pneumonia usually does not cause long-term side-effects after complete recovery.