Some white blood cells, called granulocytes, are filled with microscopic granules that are little sacs containing enzymes (compounds that digest microorganisms). Neutrophils, eosinophils, and basophils are all granulocytes and are part of the innate immune system with somewhat nonspecific, broad-based activity. They do not respond exclusively to specific antigens, as do the lymphocytes (B-cells and T-cells).

Neutrophils contain enzymes that help the cell kill and digest microorganisms it has engulfed by a process known as phagocytosis. The mature neutrophil has a segmented nucleus (it is often called a 'seg' or 'poly'), while the immature neutrophil has a band-shape nucleus (it is called a band). Neutrophils are made in the bone marrow and released into the bloodstream. The neutrophil has a life-span of about three days.

How is neutropenia defined?

The white blood cell count (WBC) is the number of white blood cells in a volume of blood. The normal range for the WBC varies slightly among laboratories but is generally between 4,300 and 10,800 cells per microliter or cubic millimeter (cmm). The WBC can also be referred to as the leukocyte count and can be expressed in international units as 4.3 x 109to 10.8 x 109 cells per liter. The percentage of the different types of white blood cells in the WBC is called the WBC differential.

The absolute neutrophil count (ANC) is determined by the product of the white blood cell count (WBC) and the fraction of neutrophils among the white blood cells as determined by the WBC differential analysis. For example, if the WBC is 10,000 per microliter and 70% are neutrophils, the ANC would be 7,000 per microliter.

An ANC of less than 1500 per microliter (1500/microL) is the generally accepted definition of neutropenia. Neutropenia is sometimes further classified as:

• mild if the ANC ranges from 1000-1500/microL,
  moderate with an ANC of 500-1000/microL, and
  severe if the ANC is below 500/microL.

What are the clinical consequences of neutropenia?

Neutropenia results in an increased susceptibility to bacterial infections. The degree of risk depends upon the cause and severity of the neutropenia, the underlying medical condition of the patient, and the presence or absence of bone marrow reserves for the production of neutrophils.

The most common type of infections seen in neutropenic patients are caused by bacteria normally found on the skin (such as Staphylococcus aureus) or from the gastrointestinal and urinary tract. Fungal infections are also more frequent in patients with neutropenia. The infections may be limited to certain areas of the body (commonly the oral cavity, genital area, and skin) or may spread via the bloodstream to the lungs and other organs in severe, prolonged neutropenia.

What causes neutropenia?

Neutropenia can be present (though it is relatively uncommon) in normal healthy individuals, notably in some persons of African or Arabic descent and and Yemenite Jews. Neutropenia may arise as a result of decreased production of neutrophils, destruction of neutrophils after they are produced, or pooling of neutrophils (accumulation of the neutrophils out of the circulation).

Neutropenia may arise as a result of numerous medical conditions:

• Infections (more commonly viral infections, but also bacterial or parasitic infections). Examples include: HIV, tuberculosis, malaria, Epstein Barr virus (EBV);
  Medications that may damage the bone marrow or neutrophils, including cancer chemotherapy;
  
  Vitamin deficiencies (megaloblastic anemia due to vitamin B12 and/or folate deficiency);
  
  Diseases of the bone marrow such as leukemias, myelodysplastic syndrome, aplastic anemia, myelofibrosis;
  
  Radiation therapy;
  
  Autoimmune destruction of neutrophils (either as a primary condition or associated with another disease such as Felty's syndrome) or from drugs stimulating the immune system to attack the cells
  
  Hypersplenism, which refers to the increased sequestration and/or destruction of blood cells by the spleen.